I just knew!!! I had AF symptoms for a couple of weeks before, suddenly became manic about cleaning the house and making lists, packing bag for hospital etc...
The night my contractions started, I had no pain, just slight tightenings that happened a bit more regularly than BX - I woke the OH and said ' I think Im in labour' but couldnt explain how I knew I was - just had a 'feeling' I was.... went downstairs and made a cup of tea - then thought 'oh I had better try and have a poo!'!! (was really paranoid about pooing during birth!!) and when I was on the loo, I heard a 'pop, pop' sound and my waters gushed out!