You'll know. Write down the times when you feel them and note how far apart they are.

You'll feel your stomach tighening up and getting hard as the uterus, which is a muscle, contracts down in an effort to open the cervix and move baby downward. If you were to press on it, if it felt hard in one area but soft in another, it is most likely baby's bottom or back pressing out against you. If it feels hard all over, comes and goes, it is a contraction.

Your uterus contracts a bit throughout much of your pregnancy, but you don't usually feel it until closer to the end. Braxton Hicks contractions are warm up contractions, or your uterus preparing. Most women start to feel some off and on in the last trimester... most often in the last few weeks. On occasion you might not feel them at first although they will be picked up if you happen to be monitored.

If you are before 37 weeks and start to notice quite a few, time them as Wondergirl said. If you are having more than 4 or 5 in an hour, try getting plenty to drink and change your activity. Being a bit dehydrated can cause your uterus to contract more. If Braxton Hicks contractions they will subside, if true labour contractions they will continue and you will want to let your midwife or doctor know. Depending on how far along you are, they quite likely will try to stop them so baby can remain inside a little longer.

If 37 weeks or later, still get plenty to drink, time them, and give your midwife or doctor a call so they are informed that things are starting. Some women will have sporadic contractions for a few days before labour really starts to begin in earnest, others will have very regular contractions from the start.

As Wondergirl said, once they get going, you will know that you are having them!