With my 1st labour, I thought it was a false alarm because it took quite a few hours for the contractions to become really painful and they didn't seem very regular.
I got DP to record the time and length of each contraction for an hour or two before I realised that they'd moved from 15mins to 8 mins apart and were regular. I counted my labour from when DP started recording.
With my other 2, I thought "Oh, this feels familiar!" and just knew, and started timing from then.
I'd suggest that once you get a couple of braxton hicks that hurt a bit, check the time and, if you're unsure, write down the time each one starts to see if there's a pattern. (Of course if your labour is progressing quickly, you'll know straight away!)