Most reliable for me: (1) Crazy distorted reflection in mirror. (2) Push a finger, or walk through a mirror. (3) Light switch.
I always look into a mirror or any reflective object whenever I see one. If nobody is around, I push it gently with my finger to see if it's solid or not. The light switch has been a very reliable one for me.

Least reliable or erroneous for me: (1) Finger counting. (2) Reading, then re-reading text.
If I'm already lucid, my finger count will most likely fail. I have a very good memory, so text usually stays the same, if I can read it at all.