How can you tell if someone has had sex?

[line writer]

I'm writing a novel, and this girl has had sex, and shes coming home and her father can somehow tell she's had sex. But how?

[Soul~Mantra]

fathers know everything

[John boy]

Fathers just know these things.

[Ron L]

how about her clothes were some how out of whack or the buttons on her blouse were miss buttoned or her clothes were different

[Malruhn]

The "sex" smell is pretty overpowering. When a girl gets all lubed up (this doesn't have to be from a guy - it can be from ANYONE or all alone!!), the natural juices have an aroma. If she's had sex with a guy and hasn't washed up, they may have been... particularly passionate... and these juices may have been spread from her navel to her knees...

Pulling up her pants and driving home, these juices will dry, and the aroma will remain. Due to her being used to them (kinda like not being able to smell the paint after you start painting - but someone walking in is nearly overpowered by the odor), she wouldn't detect them - and her dad may be able to smell them.

Also, mussed hair. Bed hair or "sex hair" can be another telltale sign.

There may also be hickeys, smeared lipstick, curled collar on her blouse, her shirt not tucked when she has a long history of tucking them in - or there may even be... other evidence. The guy may have shot off into her hair, on her clothes or anything. Her freshly pressed silk blouse may now be wrinkled - or there may be greasy hand-prints on her shirt in the vicinity of her breasts.

Or - dad may have seen her FaceBook update that said she was going to go out and get laid tonight. This actually happened to a friend... and his daughter was NOT happy when she got home and daddy was pissed off.

And, besides, dads are created to worry about our little girls. We know what kind of pigs guys can be, because we were the same way when we were kids... we look out for our girls.

[{^,^}]

I like Malruhn's answer, especially the Facebook part. That would definitely be a good reason to put in a modern novel. I just want to add that you can tell simply because of the change in mood. If it was good sex, the daughter is going to come home very happy likely. If it was bad, she might not be as happy as she was when she went out.