On the limits of rationality given flawed minds —

There is some fraction of the human species that suffers from florid delusions, due to schizophrenia, paraphrenia, mania, or other mental illnesses. Let's call this fraction D. By a self-sampling assumption, any person has a D chance of being a person who is suffering from delusions. D is markedly greater than one in seven billion, since delusional disorders are reported; there is at least one living human suffering from delusions.

Given any sufficiently interesting set of priors, there are some possible beliefs that have a less than D chance of being true. For instance, Ptolemaic geocentrism seems to me to have a less than D chance of being true. So does the assertion "space aliens are intervening in my life to cause me suffering as an experiment."

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"?