Actually, the only way to know is to ask. There are no foolproof ways to tell if someone is cheating.
Even an "eyewitness" might have the situation all wrong!
Some teens in similar situations find that confronting the person and being upfront with their feelings is the best way to sort things out. You may want to explain how you feel and ask him how he feels about the situation.
One thing to keep in mind when talking about cheating — especially when you're not sure if it's true — is to avoid sounding like you're making an accusation.
For instance, instead of saying "You're cheating on me, aren't you?" you can say something like, "I've got this funny feeling.
I hope it's not true."
Remember, any time you confront or discuss your feelings with somebody you are taking a risk. You may feel vulnerable or may get hurt in the process, so you need to decide whether knowing the truth is worth the risk.
This answer from teenwire.com
