There's not a guaranteed way to "know" if your girlfriend is cheating, but I'd like to know what has changed that makes you suspicious? How are things different?

Are the two of you still close, in general? How much time do you spend together? How many interests do you share? Do you have mutual friends, as well as separate ones? 

Rather than accusing her of anything, have you considered trying to rekindle the flame between the two of you? Don't try to smother her, but be extra-attentive. Try to recreate the best part of your relationship, as you remember it, for both of you. It shouldn't be a chore, as I would expect that both of you would enjoy being together that way.

Do remember that emotional intimacy leads to physical intimacy. If you haven't been spending quality time together, really interacting rather than just being in the same place at the same time, I'd be surprised if your sex life hasn't suffered as a result.

If you feel a need to push for an answer, I'd suggest trying to set up a session with a therapist, and speaking with her about your concerns with the therapist as a facilitator. This is a really sticky topic, and a good therapist could help both of you avoid saying things you would regret for a long, long time.

Good luck!