If so, what do you propose distinguishes between intelligence/mathematical ability and athletic ability?

To evaluate athletic ability, you use your judgment. What can you use to evaluate your judgment?

It is possible for a person to produce an accurate evaluation of a subset of their own intellectual skills, but certain skills cannot be evaluated, because presumptions about those skills are required for the evaluation to take place. You should not ask questions about subjects in which you presume you already know the answers, and you cannot ask questions about subjects where answers must be presumed in order to be able to ask at all.