Real contractions will feel like they peak and then ebb - when they start they just keep building and building and building, and then it will start to subside (usually when you think OMG can this possibly get any worse ROFL) and fade away. At first you will feel relatively *normal* in between them, but when labour is really established and contractions are very close together you more than likely wont have that feeling of normalcy in between, it certainly wont feel like the contraction just did, but you do have a bit of a lull. There isn't really much to be gained by timing them, when they start to last for longer (anything from 90secs to 2mins) and have that definate peak and ebb (whereas BH's will be a constant pain/sensation), then they are the real thing and don't worry about timing them, just go with it. using the thinking part of your brain in labour can hinder your progress.
Great point Trillian. I think my DH timed my contractions for a little bit near transition because they were just coming one on top of the other and I think he was worried about me, poor dear. But I wouldn't time contractions if I could avoid it. Every labour is different, and you may be umming and ahing about when to get to hospital etc based on contractions which aren't text book. Mine, for example, never got longer than 50secs (I had continuous monitoring), so if that had been what was helping me decide to go to hospital then I wouldn't have made it!

Advice I received on the emotional signposts of labour was the most helpful thing for me. If you can't smile, and feel really serious, you sort of sink into yourself inside and the outside world is hard to notice, that's when you know active labour has begun and should head for the hossy. If you feel like giving up and get despondent and nauseous, then you're in transition and it's not long 'til the end.