How do i know if my Girlfriend is cheating on me?

I Love my Girlfriend and i trust Her so much and because she is so amazing, beautiful and funny (and i know a lot of guys try to get at her) and i im the opposite im so afraid that she is going to find a better man and fall for him.....and i was face-book and i saw something that she told me a while back which was "Walks in the cemetery are so romantic" so i clicked on the comments and i saw some pretty funny stuff and i was okay until i saw "im gonna ask Allen to go with me" (that's what my Girlfriend put) and i know most of her family and i know that i haven't met an "Allen" plus one of her friends put "you would enjoy that you weirdos xD" (OH! and it's been a long time since we've seen each other because we ended up going to separate schools, and we try seeing each other on the weekends but either i'm busy or she's busy [im usually the busy one])