How would you know he's cheating on you?
You know if he is cheating on you through many ways.
If you don't go to the same school and his school is co-ed, it's
best to have a couple of friends, or brothers, sisters, cousins
whichever, that know him to watch him. They'll call you up and
report to you. If they all come to you and say something like a
girl was sitting on his lap or he had his arm around a couple girls
he probably isn't cheating. But if he is always around one girl
most of the time and letting her sit on his lap or hug or all that
lovey-dovey stuff, better put you're guard up and let you're
"spies" talk to him about it. Don't talk to him about it or else
he'll know you're spying on him and you don't trust him and may
result in a break up.
If he starts asking you to wear certain clothing or doing
certain things or keeps talking about this other girl he's friends
with, he could be cheating on you.
You could try checking up on him yourself if you want to. If you
know all his hang outs, try going to them and see who he's with. If
you see him getting all cuddly with another girl, that sucks for
him!! You can do what you want with him from that point, you can
either embarass him in front of his friends, wait until later to
talk to him about it, ambush him once he walks out of the area, or
get revenge on him (not my thing but you can try it).
Other than this, it could be that he's just a REAL flirt without
knowing it or just doing it for fun which could be a problem for
both of you especially if you're jealous and he just wants
Talk it out. But after you do, if you still trust him just
remember that his heart belongs to you and no one else no matter
what he does, unless he has sex or something like that with the
other girl. If you keep bugging him about it, he'll get mad and
break up with you. If you confront him about it and he acts
suspicous, don't think the worst. Oh and try not to totally throw
it in his face it'll result in trust issues and after that most
likely a break up. So be carful while your confronting him, choose
your words wisely. If he says he's not he very wel may not be, but
if you're still suspicious continue to keep an eye on him. DON'T
MAKE ANY HASTY DECISIONS UNTIL YOU KNOW FOR SURE!!!