How to tell it's labour
Time your contractions. It's probably labour if your contractions last more than 40 seconds and you're having three every ten minutes.
You've had a 'show'. This is a loss of pink mucus from the plug that keeps the cervix closed. Labour can start any time after this, although it could be as long as a few days. Some women don't notice the show, so just because you haven't seen yours, it doesn't mean you aren't in labour.
Your waters have broken. This means the membranes holding the amniotic sac, which contains your baby and the amniotic fluid, have started to leak. It can happen suddenly, but is more likely to be a gradual trickle. Again, you may not notice your waters going at all. It's more likely to happen when you're already in established labour. There's a risk of infection if your waters break too soon before labour and the waters could bring the cord down as well, compressing your baby's oxygen supply (this is rare). Call the hospital if you're concerned.