How do I find out if my girlfriend os faithful to me?

You could play the "I got you" game with her.
Example:
You: Hey, I saw you at the mall with another guy
Her: What are you talking about?
You: Don't lie to me, I know you're seeing someone else
Her: What?
You: All right, just tell me his name
Her:?
You: All right, I'll give you some time, I will now smoke a cigar
Then you exit and leave the house/department for a while. Two things may happen. She will try to stop you and confess or let you leave with the w.t.f. face. When you come back ask her okay, now tell me. NOTE: LOOK STRAIGHT TO THE EYE. And ask, is it true? She will finally confess, tell you that you're an or lie and tell you she is faithful to you.

OR you can just hire a private detective. OR ask a friend your girlfriend doesn't know to spy on her
Good Luck, and tell me what happens :)

you could play the "I got you" game with her.

Now I understand why you stay in your room and have lost faith in women and relationships.

Just ask her and when she answers look at her body movement. When a lot of people lie they move around a lot.

Anouther way to tell is if she gets a phone call and leavs the room so you can't hear it.