There is no way to positively know if your boyfriend has cheated on you unless you catch him or he tells you. There may be some signs such as not being where he says he is or lying to you on who he was with.
You just ignore it because you know you didnt cheat but if you did, that's a whole diffrent story.
you might be getting cheated on ):
HOW CAN YOU STILL LOVE HIM AFTER HE CHEATED BREAK UP. HE ONLY SAYS HE LOVES YOU TO AVOID THE WRATH Belive me I know
The only thing you can do is just move on if he cheated on you with your best friend. Don't be mad at her, it's not her fault. It is his fault he was not a true boyfriend after all.
You'll have to take his word for it. Unless he gives you a reason not to trust him, you have to. Don't go looking for trouble.
This question doesn't make sense. Find someone who doesn't cheat, first and foremost.
You can't tell 100% sure. Talk to him about it, but you have to have enough trust in him to believe what he says. If you don't have that much trust in him, then you may want to reconsider being with him. It's a hard thing to have a relationship with someone you don't have full trust in.
If mean she/he wants you as a girlfriend/boyfriend
There are two possibilities: one is that she is lying to try to break you up or just to be mean, or she is telling the truth and he is lying. Regardless, I think it is safe to say she is not someone you should trust automatically, as she must be either a cheater or a liar.
he probably is mad at you then.
you will have to earn the trust back and it will always be harder for him because he wont know if you will do it again.