One other way you may be able to tell is to look closely at the paint job. VC-17 is a very thin paint which does not build up and require sanding so what you see as texture on the paint is pretty much a mirror image of what is below. Although the barrier coat requires some roughing up prior to painting, you should still see somewhat of a textured look to the surface left by a roller or brush. The VC will be nice and smooth but you can see the difference between that stuff on gelcoat and on barrier coat which will most likely be rougher. Also, you can take a dab of acetone and rub a tiny spot of that VC off in a second to see what is below. Then touch up with like paint.