Before getting into a relationship with a girl how can you tell whether she is likely to be faithful?

You can't ever be certain, just as she can't ever be certain that you'll always be faithful to her. Generally though, if she has moral values, respects herself and can follow through and commit to friends and family, the risks of infidelity are less, but people change. Don't try to be suspicious or attempt to figure out that kind of stuff. The best way is to not limit your options. Meet a lot of people, possibly at a religious function, school or work. Find out about the girls you like, what makes you think that someone would be good for you in the long run. "Loving" someone is easy. Finding someone you can "Like" for the rest of your life is difficult. If you can be someone that the other person can "like", you will have a much better chance at a long term relationship.