Well, in principle you can determine sources and sinks of a flow by looking at its divergence. Regions with divergence larger than zero are sources, smaller than zero sinks. This is valid for a 3-D flow in 3-D, or a 2-D flow in 2-D - you, however, have 2-D images of 3-D flow so extra caution has to be taken. But since we're already on our way - you might get something out of trying to look at divergences of your optical flow, you most likely should do whole lot of low-pass filterings on the divergence and perhaps the flow-vectors.
If that fails you could have a look at Helmholtz decomposition and further on, if you can get some more robust estimates.
Let us know how you proceedings go. Good luck!
HTH
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