How do i know if someone has a sexual transmitted disease?

They tell you: Most often, the people need to be tested for std's and then it is really there moral and ethical obligation to tell you they have an std if you are a sexual partner of theirs. Simple clues of course could be lesions on the genitals, a foul smelling discharge, etc. But you would only suspect it, and then should avoid sex until you know what your getting yourself into. Best wishes.

STD: Most of the time you can't tell whether someone is infected. You need use condoms all the time in order to protect yourself and make sure both you and your partner get tested. Take care!