I'm serious; one of the reasons I don't engage in casual sex with men is because I would have to trust his self-report of whether or not he has HIV (and almost half of the men with HIV don't know that they have it).
Did my quantified risks post make you more or less concerned?
A post from Gregory Cochran's and Henry Harpending's excellent blog West Hunter.
The commenter Ron Pavellas adds:
The Wasserman Test.