The best way to tell is to talk to a physician. However, a simpler way is to avoid eating milk products for a several days and then eat something very milky. I didn't think I had a problem, but had avoided milk for my boyfriend's sake. Then one day I had a bowl of ice cream, and was in agony.
It doesn't always have to rule your life. Sometimes I can tolerate small amounts, and if you're careful you can to. With the help of enzyme aids like Lactaid, you can enjoy milky treats every once in a while.