how would you if your partner is cheating on you

Usually when someone is cheating there may be a change in their behavior and how they act towards the girlfriend/boyfriend. There can be a change in intimacy or the partner may become non caring or even mean. Either way if this guy is cheating she did the right thing to leave him. If he is cheating on her he really doesn't love her and she will find someone else that can appreciate and love her. That's just my opinion.

It is when the boyfriend is giving you less time and less enthusiastic about the relationship. He will most likely take things for granted in a sense that what is important to you, may not be important to him anymore. He won't let you read his cellphone. He has two cellphones, one for you and one for his new girl. He comes home late at night without telling you where he went. Those are just some of the signs. Keep your guard up!

I'm not really sure about that since some people are good in hiding things. I think if they don't answer the phone most of the time or won't reply on your messages (if they are not at work), that could be a sign. Or if there are times that you would love to go out with them and they don't seem to be available. Well of course there could be a lot of reasons why they are busy but they should have let you know ahead of time.

people act subconciously different.

It takes a lot of instinct and a lot of willpower to confirm your instinct to know if your partner is cheating on you.

If it has been confirmed that my partner has been cheating on me for such a long time then there is no point in trying to hold on to her. It is better to leave her and move ahead in life. I know it is not easy, it will give me a lot of pain but that is the only solution. I would never even consider giving her a second chance because I am a human being, I am not a Saint and I have self respect.