Before discussing any of the technical issues associated with your question: Yes, in order to ensure that a statement (about numbers) is true (in the standard model), we currently do not have any procedure other than exhibiting a proof. (In fact, most mathematicians would argue that this is the only way to meaningfully discuss truth.) If the statement is unprovable in $\mathsf{PA}$, such a proof by necessity is in a stronger system, typically a fragment of set theory, $\mathsf{ZFC}$, sometimes a fragment of second order arithmetic, $\mathsf{Z}_2$. But sometimes we go beyond $\mathsf{ZFC}$. The typical extensions we consider usually assume large cardinals. There is a well justified body of evidence indicating that it makes sense to say that proofs of arithmetic statements in these systems are indeed true (I link to a small discussion on these matters near the end).
(Since proof is at the heart of this discussion, you may be interested is a series of essays by computer scientists, mathematicians, and philosophers, on The nature of mathematical proof that was published in the journal Philos. Trans. R. Soc. Lond. Ser. A Math. Phys. Eng. Sci, vol 363, No. 1835, Oct 15, 2005. Sadly, the only link I have is behind a paywall.)
There is a technical difference between the sentences produced by Gödel's techniques (or Rosser's) and sentences such as the strong Ramsey theorem that the Paris-Harrington theorem discusses. (For additional examples, see here and the references listed there.)
Gödel's statement is $\Pi^0_1$, that is, it has the form $\forall n\,\psi(n)$ where $\psi$ is a recursive statement (in the language of arithmetic, all its quantifiers are bounded). The strong Ramsey theorem is $\Pi^0_2$, that is, it has the form $\forall n\,\exists m\,\psi(n,m)$, where $\psi$ is recursive.
$\Pi^0_1$ are meaningful statements even by strict finitistic standards (see for example Richard Zach's dissertation and his discussion of Tait's analysis of Hilbert's program). They are true if they are unprovable (in fact, $\mathsf{PA}$ proves every sentence $\phi$ that is true of the natural numbers and such that $\lnot\phi$ is $\Pi^0_1$): If $\exists n\,\psi(n)$ is true, then for some number $m$, we can prove this statement simply by verifying $\psi(m)$ which, being a recursive statement, has a straightforward verification -- informally, we run a computer program that is guaranteed to halt, and check that the program outputs $\mathtt{Yes}$.
On the other hand, $\Pi^0_2$ statements cannot be called finitistic. If unprovable, this does not automatically ensure their validity (or falsehood). However, they admit a natural interpretation: They state that certain recursive function is total (if the sentence is $\forall n\,\exists m\,\psi(n,m)$, the function $f(n)=m$ assigns to each $n$ the least $m$ such that $\psi(n,m)$), that is, they state that certain computer program will halt, no matter its input. Typical independent $\Pi^0_2$ statements have the peculiarity that the corresponding recursive function is fast growing (see for example this discussion, or my article on Goodstein sequences). By a theorem of Wainer, there is a standard mathematical approach to verifying their unprovability, namely, one checks that the function $f$ under discussion eventually dominates the first $\varepsilon_0$ functions in a fast growing hierarchy.
One could consider Gödel statements and the like as first generation independence statements, and the more "combinatorial" $\Pi^0_2$ statements that followed as second generation. The mathematical arguments involved in either case are really very different. A third generation family of independence statements would be $\Pi^0_1$ combinatorial statements, so they are finitistic in the sense of Hilbert's program, but they have immediate, apparent mathematical (as opposed to meta-mathematical) meaning. Such a family of statements is considered of great interest. Remarkably, recent work of Harvey Friedman has produced such examples, see for example his book on Boolean relation theory, available at his page.
This is by no means the end of the story. For example, we now have a whole family of independent meta-mathematical statements of increased complexity of which $\mathrm{Con}(\mathsf{PA})$ (the sentence in the second incompleteness theorem) is but the weakest possible. See for example Aspects of Incompleteness by Per Lindström, or Metamathematics of first-order arithmetic by Petr Hájek and Pavel Pudlák. In the context of set theory, we can go further, as the completeness theorem tells us that set theory is consistent if and only if $\mathrm{Con}(\mathsf{ZFC})$ holds. We can go beyond this and require the existence of an $\omega$-model (one whose set of natural numbers is isomorphic to the standard model) or even stronger, the existence of a $\beta$-model (that is, a well-founded model). Woodin has explained how his $\Omega$-logic provides in a sense an ultimate extension to this process.
We also have families of $\Pi^0_2$ statements whose truth is harder and harder to verify. For example, the Paris-Harrington theorem gives a result that is not only independent of $\mathsf{PA}$ but also independent of the theory obtained by adding to $\mathsf{PA}$ all true $\Pi^0_1$ statements. But it is easy to verify once we go beyond this theory. On the other hand, Friedman identified a finite form of Kruskal's tree theorem that cannot be verified by so-called predicative means, which means that adding it to $\mathsf{PA}$ gives us a system with consistency strength significantly higher than that of $\mathsf{PA}$ with, say, Goodstein's theorem.
And we can go beyond, as Friedman has examples of $\Pi^0_2$ statements whose proof requires assumptions of a large cardinal character (typically, the existence of Mahlo cardinals of all finite orders, but recent examples go much further).
In set theory we study the consistency strength hierarchy, and have identified remarkable structure, starting with the identification of the large cardinal hierarchy. For a brief and somewhat technical discussion of these issues, see here. The point is that we can see these results as extending significantly beyond arithmetic the realm of what is true but unprovable (in specific formal systems).
Best Answer
The crux of the matter is that (people claim) we have evidence that PA is consistent, but we do not have similar evidence that CH is true. Note that "true in the standard model of set theory" is (basically) synonymous with "true."
Why is this? Well, let's begin with: why do we believe PA is consistent? Usually, actually, a stronger assertion is made: PA is true. The reasoning behind each claim is often, "We have intuitive access to the natural numbers, and this includes the knowledge that they satisfy PA." (If this sounds circular to you, don't worry, you're in good company.)
Now let's leave aside the issue of how convincing or not our ability to visualize the natural numbers is as an argument for the consistency of PA, and look at CH. First, note that we arguably know how to convince ourselves that PA is consistent: all it takes is the ability to find a single ordered semiring in which PA is true, and what goes on in other ordered semirings doesn't matter. By contrast, if we had a model of ZFC+CH, this would only be evidence for the consistency, not truth, of CH; in order for the existence of a model $M$ of ZFC+CH to count as evidence for the truth of CH, we would need
a reason to believe that $M$ is the standard model of set theory, or
a reason to believe that $M$ is similar to the standard model of set theory, at least as far as CH is concerned.
This difficulty is compounded by forcing, which lets us explicitly build a model of ZFC+CH from a model in which CH fails, and vice versa, while preserving many nice properties (such as well-foundedness). This (in my mind) kills off, for example, the hope of arguing that there is a single model of set theory which is somehow "within reach": simple models have simple forcing extensions.
So now on to your first sentence:
Here's one approach: identify mathematical properties which, according to some philosophy, the standard model of set theory must have; then, show that these mathematical properties imply/disprove the statement in question.
For example . . .
There are arguments that the standard model of set theory satisfies "V=L"; insofar as you buy the philosophy behind these arguments, these are also arguments for CH being true. However, they tend to be unpopular.
Large cardinals are very "in" these days (:P), but they don't settle CH (although they do imply, for instance, projective determinacy, and so many set theorists believe that projective determinacy is "true in the standard model").
Forcing axioms - such as PFA - imply that $2^{\aleph_0}=\aleph_2$; on those rare days when I believe in the standard universe of set theory, I tend to believe in this direction, but I think that might be rarer (more common is the belief that forcing axioms hold in inner models; this is basically large cardinals round two).
Woodin has examined some other means of settling CH, but I know less in this direction; basically, one of his approaches ("Ultimate L") is to argue that, assuming large cardinals hold in the "real" universe $V$, there is an inner model $N$ which is "large" (i.e., has the same large cardinals as $V$) and has many nice canonical features, including CH. One can then make arguments that $V$ "ought to" be equal to its own $N$.
For more and better information on these and other arguments, see https://mathoverflow.net/questions/23829/solutions-to-the-continuum-hypothesis.
EDIT: The lectures etc. at http://logic.harvard.edu/efi.php#multimedia might be of interest to you; they discuss the nature of mathematical truth, the definiteness of mathematical statements, and whether CH has a truth value.