How can you tell labour has started?

This is probably a question millions of women ask, but for good reason! As a first-timer it just seems so alien a concept that pretty soon at an unscheduled time and place, labour will just begin. The description of what  a contraction feels like also seems so vague. When speaking to my friends that have been through it, everyone's experiences seem to have been so different.

I am in the unfortunate position that my baby is 'back to back' or in the posterior position. As a result my back pain is pretty hideous most of the time anyway, but today it went to a new level and was accompanied by some pretty random and strong pelvic pain. Could this be it I ask?

The answer is probably not. The pain has stopped now and I feel a bit silly for thinking this might have been it. 

Surely once you are in labour you know? Fingers crossed it will start soon. :)