I've personally found
indicators (such as, it always
always begins in the room that you're meditating in, and you'll feel vibrations,) quite consistent in themselves... however, what about false awakenings? That's a dream that you wake up. It usually starts in your bedroom, so the former isn't exclusive to astral travel even if I have personally found it consistent. As for the vibrations... I actually read somewhere that they're supposed to disappear after a while unless I'm doing it wrong. ;P Like bellyflopping instead of diving into the pool every time. So, when there are no vibrations... how can you tell?
The only real way you can really tell, is if you can verify with somebody else if something happened the way you saw while out-of-body, or if another projector can verify a shared astral experience. I haven't been able to do any of that myself, so I sometimes post in doubts about there being a line between dreams and astral projections, but a part of me nags an explanation that the rest of me finds annoying and unacceptable: "I just know."
Hope that helps. Somehow.