Impossible. First, condoms don't protect from skin contact around genitals, and you are guaranteed to have various forms of genital herpes. If you aren't covered with pimples it doesn't mean anything other than that for these particular strains your immune system is effective enough to not have something visible. Then we have diseases like various forms of hepatitis. For example you can be infected with hepatitis B, and it could be suppressed by your immune system to the imperceptible level, basically defeated, laying dormant, until your immune system weakens... There are numerous diseases that can be spread that way. HIV included.