I might have limited friends but the way to finding true friends is what I have resting under my belt. Here goes nothing.
Way Number 1.) If the person is the opposite gender of you (runs more smoothly if you are a girl and the attempt for a friend is a male) simply go up and feel their hair. If they have no problem and shrug it off that's a sign of friendship. On the other hand if they say cruel words and run or hit hand away that could be an issue... Just could.