Whats the best way to find out if you are related to someone?

Ok, here's the situation. Me and my girlfriend have been dating off and on since we were in the 7th grade. We're now in the 11th. I havent met her mom yet but she knows who i am...we're going to prom together this weekend.So when she tells her mom she's going with me, her mom says that we are SUPPOSED to be related. But the problem is, we don't know for sure or even how we are SUPPOSED to be related. I've asked my mom & she said she doesn't know if we are related to them. I've even asked my Grandmother & Great Aunt. They both said they thought we were but they don't know how. Will someone please help me? I can honestly say I dont know what i would do if I found out we are cousins.Me and this girl are so in love.And we can't imagine letting each other go.