In a symplectic manifold $(X^{2n},\omega)$, a hypersurface $Y\subset X$ has contact-type if there is a contact form $\lambda$ such that $d\lambda=\omega|_Y$. Recall that a contact form is a 1-form with $\lambda\wedge(d\lambda)^{n-1}>0$, i.e. the opposite of a foliation. For example, a starshaped hypersurface has contact-type in $\mathbb{R}^{2n}$, or more generally an $Y\subset\mathbb{R}^{2n}$ transverse to a Liouville vector field defined in a neighborhood of $Y$. In particular, any contact manifold $Y$ is a contact-type hypersurface in a symplectic manifold (the symplectization $\mathbb{R}\times Y$).
Now it's nice and useful to consider symplectic manifolds where its boundary has contact-type. And this can usually be done given a compact symplectic manifold (cobordism between contact manifolds).
This leads to the question of whether or not you can always build such a space. In other words:
Given a random hypersurface in $\mathbb{R}^{2n}$, is it of contact-type? How do you tell when it's not of contact-type?
Edit: In light of the posted responses, I think it would be appropriate to tweak one of the questions above. In particular, it was essentially pointed out twice that in the "set of hypersurfaces" there are open neighborhoods which contain no contact-type ones. But, are "most" hypersurfaces in $\mathbb{R}^{2n}$ of contact-type? i.e. Should I expect my hypersurface to be of contact-type?
Best Answer
Not all hypersurfaces in $\mathbb{R}^{2n}$ are of contact type.
Weinstein, in the paper: "On the hypothesis of Rabinowitz' periodic orbit theorems", where he defined the concept of contact type, gives a criterion. If $H^1(\Sigma)=0$, and $\Sigma$ is of contact type, then the characteristic line bundle comes with a distinguished orientation, determined by those vectors $\xi$ such that $\alpha(\xi)>0$ for all contact forms $\alpha$. This is independent of the contact form. For periodic orbits this induces a positivity criterion. In the same paper he also constructs an hypersurface in $\mathbb{R}^4$ which is not of contact type, by showing that the criterion is violated.
Many hypersurfaces are of contact type, as you remarked. Another nice example are mechanical hypersurfaces. These are hypersurfaces arising as level sets from hamiltonians $H=T+V$, where $T$ is the kinetic energy term $T=\frac{1}{2}\vert p\vert^2$, and $V$ is a potential depending only on $q$. This works in general cotangent bundles.