Okay guys lets be more blunt and honest here when answering this question. In my own opinion, one way that you can tell if this person is cheating is by his or her action. Watch and pay attention to everything he or she is doing and saying.
If this person is cheating, he or she will be uncomfortable around you. When he or she talk, he will be more careful so what ever is going on behind your back will not be given away. People who do cheat will act and talk funny because they don't want to get caught. If they are not a talkative person with you and all of a sudden loves to talk, that will be one sign.
What you need to do is pay attention to everything he or she do. But I think it will be much better if you talk to this person who you think is cheating on you. This is the only way that you can find out if something is going on.
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