How to know if cat had sex?

There is no way to tell. The only thing you can do is call the vet first thing Monday morning and get the first available spay appointment for her. If you can't afford the vet's prices, then call your local shelters to see which ones have a low-cost spay program. She needs to be spayed ASAP, as if she did mate she's certainly pregnant. Getting her spayed eliminates that possibility now and in the future.

Did she come back smoking a cigarette? No seriously.

My guess would be if she was in heat and went outside she may have had sex, but sex between cats is often very scratchy, she would likely look like she got in a bit of a fight (you know, the same way people look after really, really good sex, a little beat up)

Well cats have a sense on where "home" is so don't worry about her being lost. As for the pregnant part she will start to get a bigger stomach start eating more etc. If she is pregnant don't have the baby's killed simply just keep them or sell/give them away.

If she's laying on her front with her butt in the air and meowing/ howling then she's in heat and will be pregnant, if she wasn't in heat she can't get pregnant because she will fight with the male n wnt let him do it. If she's in heat check the back of her neck if the fur looks thinner than usual or if bits are missing then she's been mated, the male climbs ontop of her back and bites the back of her neck to hold his balance and hold her still she will alow him to do this but obviously fur will be missing.

get her fixed asap.... seriously u needa get ur cat fixed there are millions of stray cats born each year and countless suffer and die. if shes prego the fixing will take care of it -- itll be like plan B lol.. or an abortion.

If your cat had not had a virginity already, then you cannot to check it now.