text: Apparently, you know how to tell if your partner had sex, because you are asking this question and deep down you already know, but you have turned it into a puzzle to be solved -- even though you already know.  \n\nWell, anyway, if your partner is a man, then you can tell by having sex with him as soon as possible after you suspect he had sex with another woman and then seeing how much semen he has... and compare it to another time when you had sex with him and you are sure that he didn't have sex for 2 days.  Of course, this is not a 100% sure test since he may have masturbated instead while he was away.  But, in either case, you suspect him and that is a bad sign for you in this relationship.