Obviously it's a touchy subject and a little rude to just come out and ask a woman, but do providers typically post something like that if they have one or disclose it at some time?  Is there an appropriate time/way to obtain this information?

Responsible providers do have frequent examinations, (And so do responsible clients.) and if they do test positive for anything they stop working until they are cured; but there is no way that a client can find any of this out.

Each client has to make a determination as to whether health concerns are so primary that he (or she) will either not participate, or if they do, just how risky their involvement will be.

It's just the nature of the beast.