If there were a solution that could be made at home to detect the presence of semen one could find out if there daughter were having sex.
unless the daughter was raped, there is no probable cause to test for semen. EDUCATE the daughter about sex and its threats (STDs, pregnancy, emotional dependency on the partner, etc). Its never a good idea to treat a child like they're a suspect of a crime, doing so only puts a gap in your relationship with her.
Lol. You made my day. You'd better hope that you dont find semen even if she is having sex. You cannot test for semen. You can check for pregnancy and STDs. Look, if she's going to have sex, then she's going to have sex. At least make sure she's protected with both birth control and a condom.
I suppose you could shine a blacklight on her crotch every time she comes home.