How do you know if a girl is going to be true and devoted to you?

There's no quick way to tell this, or any other good quality in a person. The best way to know is to get to know a girl as a friend for as long as possible. Watch how she lives her life. If you can't wait it out as friends, be prepared to live with the consequences and "gamble" as the first guy said. Dating is fake, friendship is real- it's the very surest way to know what someone is REALLY like. Then you'll see clearly what she does for you when the chips are down.