Okay Let’s say that, hypothetically, your partner was out all night and you suspect that he/she may have slept with someone else. There 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.
You get the point.