Is there any trait or way to determine if a Pastel is a super pastel before breeding her? I assume no, but I could be wrong...
Pastels can be extremely variable and there are some cases where it can be difficult to tell the difference between a really nice pastel and a not so great super pastel. The best time to tell is right after hatching where the supers are quite a bit lighter, especially on the head. But as an adult, it may be that the only way you can tell with certainty is to breed it and see if you get 100% pastels or some normals as well. If you get any normal babies, then the parent is obviously not a super pastel.

Mark