There's a thread called 'your first conscious projection' that is stickied in the OBE forums, I suggest you give it a read- it lists every possible symptom you might get of trance. It isn't a 'piecemeal' type of thing, because some symptoms indicate you're ready to try to exit, but they're not the same for everyone. When I was learning I found that when I heard voices I was ready, later on it was strong vibrations, and later even, there had to be a combination of symptoms. So it's a question of waiting long enough and experimenting, changing what you consider 'the symptom' as you change with your practice.