She tells you.
Someone you trust tells you.
You catch her.
The person she is cheating with tells you.
Otherwise you do not. The so called 'giveaway signs' are not proof, there may be other reasons for them.
If in doubt ask.
Follow her
tell her.
then he must be cheating on you if he has a girlfriend? i mean if he likes you and has a gf then he must be cheating and wanting you?
We can not know that and thus can not provide you with an answer.
Ask her if shes cheating on you! Nah im kidding i dont know sorry ? :( and im a guy
She keeps her phone and diary away from you.
cheating is doing anything you wouldnt want your wife/husband or girlfriend/boyfriend to know about
Sweetheart, if you who know who she is and possibly lives with her don't know, we certainly do not.
if your girlfriend is out and you don't know were she is you should probibly look for evidence that shes not cheating on you
If you have a girlfriend and a boyfriend, then you are cheating on both of them.
On a holiday, you would want to spend time with your girlfriend. And if she is cheating, she'll refuse to show up. That is one way to find out. Also, her weird reasons for not doing what you usually do, can imply that she is cheating.
tell your brother that you know that his girlfriend is cheating on him and if he doesn't listen to you he'll eventually frigure it out