O.K. Let's say that, hypothetically, your partner was out all
and you suspect that he/she may have slept with someone else.
is a product out called Checkmate
that can be used to test clothing for the presence of semen.
This is generally done by taking the panties or underwear that
he/she was wearing and testing them. You could also test a
suspicious stain on any article of clothing, car seat, whatever.