Rug burn is a great way to tell if a girl is a slut.
check knees, lower back, and elbows. now be aware, some girls are in sports that merit rug/wood burns, but some arent, dont be afraid to ask your new girlfriend where she got them, and if she says she was drunk and tripped, shes lying, unless she was drunk and tripped, and got banged like a hammer from behind, in which case she still wasnt totally honest.
a younger friend of mine brought a girl over the other night who had some gnarly scabs on her knees, and couldnt rememer how she got them.... just figured all you highschoolers needed the 411.
and feel free to share your rugburn stories, i will if it is justifyable (epic claim)