How do you know if a guy is sexually active?

There's no physical way to tell. He has to volunteer the information.

He's happy.

Actually if you check his underwear there maybe semen / vaginal juice stains if he didnt shower after sex before getting dressed. Also maybe matted pubic hair with dried fluids.

If you are willing to get more and more forensic there continues to be small clues. For example if he hasn't yet urinated there continues to be semen in his urethra. The Prostate and cowper organs will be deflated somewhat. In the Epimethius and Vas Deferenz there will be a lack of mature sperm.