I was really into it in my early 20's. One way to tell if you are mentally in the state to lucid dream is if you no longer feel tired. One night, after a grueling hike, I was completely exhausted when I went to bed. I closed my eyes, and moments later all my exhaustion just vanished, and I began to explore the space.

Another way is to try to see what the clock faces say in your dream. Also, see if the light switches behave as you would expect.