Answer:
Gonna answer this from the perspective of someone who is a former self harmer.
1. Withdrawl from social settings. (I stopped talking to friends / family around the time that my self harm was the worst because I felt like if I did that my secret wouldn't be exposed)
2. Long Pants / Sleeves when its hot out - maybe along with the claim that "Oh I'm always cold." (No. We aren't cold. We're burning up. But we can't risk someone seeing our arms / legs when we've been self harming)
3. More anxiety / depression / anger issues. (These are all heightened while self harming, and sometimes even for awhile after we've stopped.)
4. A change in sleeping / eating habits (I started to sleep and eat less, while also having days where all I did was eat because for the past week I only really had one meal)
This isn't a full list and obviously different signs present in different people. If you suspect a friend is self harming do NOT be confrontational about it. You can bring up the topic in a friendly way and if they deny it to you then don't press on. You can bring it up again, just try to keep it at least a week apart.
It's frustrating to deal with someone who is going through this, I know, but remember that they have feelings as well.
It's best to deal with them as you would a child. Take things slowly and don't get upset or yell when it comes to these topics.
But also - If they do open up to you about their self harm, do as they wish for you to. Don't tell a parent or a counselor even if that seems best because it can also make it worse for them. Respect your friends wishes and don't treat them any different. Just be there for them and support them.