Whatever it is, it has to be something that you do consistently. So, for me, it was holding my nose so I couldn't breath. If I was actually asleep, my hand wouldn't actually be pinching my nose and I would be able to breath. That and for some reason clocks reset themselves the moment I look away from them in a dream. That don't happen in real life.

The big thing though is to keep dream journals. This one is pretty big early on, as forgetting your dreams is the worst if you are trying to go lucid.