What you are experiencing is probably Braxton Hicks. If they are irregular, feel like a period cramp, and only part of your stomach gets hard it is probably BH. If you are getting a sharp shooting pain starting in your belly and going down through your vagina, and you are not able to walk and it takes the breath out of you, that is more than likely a contraction.
If you lay on your left side and drink water and the cramps get stronger, that is also a way to tell if it is a contraction or not.