If I believe that a belief B has a < D chance of being true, and then I receive what I think is strong evidence supporting B, how can I distinguish the cases "B is true, despite my previous belief that it is quite unlikely" and "I have developed a delusional disorder, despite delusional disorders being quite rare"?

The basic idea is to talk about your belief in detail with a trusted friend that you consider sane.

Writing your own thought processes down in a diary also helps to be better able to evaluate it.