How To Tell If A Friend Is Self-Harming

I suspect a friend is self harming. How can I be sure?

You can't really be sure. It depends what kind of friendship you have with them. If you can talk about this, then you could ask them in a non accusational way and just maybe say you're quite concerned, and that you've noticed that they have a few cuts or they're not taking care of themselves. Is there something they wanted to share with you and create a safe place. Or you could encourage them to call Child Line for support or talk to a trusted adult for support.