Like others have said, the only way you can know is by asking her. Yes there is a chance she will lie, but here's a quote "Loving someone is giving them the power to destroy you, but trusting them enough not to."
Of course, this doesn't mean if she says "No.. I'm thinking about dating this other guy" you should let her walk all over you.
If she says yes she does love you, stick it out. If she says yes I do love you, but I am thinking about this other guy.. decide for yourself if you want to stick it out or not. If you are young... most likely you will have crushes on other people even if you are in a caring relationship. Some married adults have crushes on other people, but they know it's nothing but a little crush and is nothing compared to the love they have for their husband/wife. So you choose if this is acceptable to you or not.
If she says she doesn't love you, and is thinking about another guy, take it as is, and let her go. It may seem like the world would end if you do this, but in the long run you'll be happy. I had a boyfriend when I was younger I was with for a few years. I thought I could never love ANYONE like him, I thought I'd die if he left me. We were on again off again and when we finally broke up for good it was the end of the world to me. I couldn't eat sleep or anything for a couple months. It was HORRIBLE! BUT I did get over it. I moved on. Now that I found my fiance, I realize I didn't love him, I was just young and in "like" with him. I thank God we ended, if not I wouldn't have found the man of my dreams
