For me it must have been the point where I suddenly declared I was getting out of the bath. and did so, completely naked and tried to walk accross the hall back to my room - naked
DP threw my dressing gown on me (thank goodness). My head just suddenly realised that I had to get up and about, I don't really remember too much but I do know I suddenly had one clear thought - that it was time to stop messing about and get DD out.
I got into the room, got on the platform bed thing, grabbed a bean bag and declared I was going to start pushing.
She arrived 45 mins later
I did not realise it was transition though. at the time it was just a change in mind set.