odds are if you have a friend who is self harming the usual way it is done is cutting. If you touch their arm they may flinch because they have cut there. I have had no experiences in finding out that my friends have self harmed unless they tell me about it after they have already stopped. If i were you i would ask/talk to them about it but don't push too hard. I wish whichever one of your friend whom do good luck and you to.