Everyone worries about this hon. If you are lucky, your waters will break, so you will know for sure! Otherwise contractions will start, and will get more and more frequent over time, getting more painful and lasting longer. Its hard to explain how they feel, mine started in my back and spread to my bum and tummy, and felt like a tightening.
It might take an hour or two until you know for sure, as sometimes you can get contractions that start an stop.
Nicola xxx