To be honest, I didn't know I was in labour because I get really bad period pains, so I thought it was just braxton hicks (practice) contractions. But you get kind of a crampy tummy right down the bottom which lasts for a few seconds, then goes away. Then might not happen again for quite a while, mine ended up being all over the place, 15mins here, 30 mins there, but closer to giving birth they get closer together and last for longer. Everyones water breaks at different times, but I think that the majority of the time it's not like on TV where a woman is in a shop and suddenly her waters break all over the floor - I'm told that is very rare. You might get what they call a "show" in your pants before hand which is a little bit of blood and mucus or spotting. But I think it can happen a week or so before sometimes (not sure), my show was only about 5 hours before my son was born. The one thing for sure is that when you need to push, you'll know, it pretty much feels like you want to go for a poo!
Hope this helps you a little bit.