contractions

newyorkchic5

I am 36 weeks along and was wondering how to tell when im having contractions. I know my stomach is supposed to get hard and they will be painful as they progress. My problem is i am a very small build. I am 5' 1 " and weighed at most 105 to begin with. So my stomach at this point is hard 95 % of the time and by hard i mean it always feels at least as hard as my chin. Sometimes it feels a little harder but i can feel her move when it gets harder. I am a ftm and just really don't know what to look for other then the obvious which doesn't work so will in my situation. Any insight would be greatly appreciated