What if you run JUnit tests on a JavaFX project with Cobertura?
Well, clearly Cobertura can't handle the syntax of JavaFX, so I think many figures are wrong (e.g. Branch Coverage, Complexity; I'm unsure about Line Coverage). But some bit of useful information is here, at least for what concerns what you haven't tested at all.
You get also some file-level details, but the incapability of understanding JavaFX syntax lead to inconsistencies (note red lines with a non zero count). But it seems that searching for non zero counts is a good way to find out which lines you don't have covered).
Who knows whether anybody will care about writing a Cobertura plug-in for JavaFX? If Sun has got a few money for pushing small projects, it would be a good idea IMO.