Still, is there a test, other than zooming in, to tell whether a point is on a surface, instead of just really, really close? If not, how do I know I'm zoomed in enough during inspection?

E.g., as far as I can tell, the end of the curve in this attachment is on the surface, but I still can't open the end of the tube on that surface (I've left it un-unioned to make it easier to inspect).