I check myself to see if I feel achy, or gravity It's hard to explain, but in my dreams, I don't really feel my body... so I can instantly tell when I'm dreaming, based on that feeling alone.

That said, this doesn't get me lucid... it's merely an infallible reality check (for me).

How often do I do it? It's hard to say, I don't count. Whenever I think about it. I think about dreaming a lot, so it definitely coincides with that.