Make her a shirt that says "I Partied With Amanda Todd"

Ask her if she cuts. Then quickly add, "the line at school". Her reaction should tell you everything.

Well to be completely honest, I struggled with self harm for 10 (almost 11 years) due to bullying when growing up. So honestly, things you really want to look out for are if she's always wearing bracelets or long sleeves. Many cutters will cut in these area; Wrist, shoulders, arms, stomach, thighs and sometimes chest. If they wear long sleeves on hot days or long pants on hot days it's something you should watch out for. If they wear bracelets a lot or put their hand over the area their bracelets are on also could be a sign. Also, locking themselves in their room or bathroom for long periods of time or shutting themselves out to the world can also be a sign. Not sure how old she is but she could be getting picked on or made fun of or she could just be dealing with issues such as self hate towards herself. If you notice any sharp thing missing from around the house or razors or sharpeners with no blades in them then that's also a concern because those are the two easiest ways to get razor blades. If she has things like facebook, youtube or tumblr, I'd suggest watching out for those to see what's being posted. Also, pay attention to the kind of music she listens to, if it's a lot of depressing music, talks about death, suicide, loneliness, etc. Self harm leads to a lot of things; depression, suicidal tendencies, anorexia, bulimia and more. When I struggled with it, being a guy, I would cut my wrist, hide them with bracelets, burn my shoulders or cut them because my sleeves of a shirt would just cover it, or my thighs because male shorts are usually longer than what females wear. So you have to see if there is a change in how she dresses. I hope that helped.