I have had five lucids (including one in which I woke up instantly) since finding this site.

The first one just happened spontaneously.

In the second one I suspected a dream because my body was moving oddly as I walked, did an RC (looked at my hand) and bang!

In the third one I noticed that my friend had huge elf ears and I instantly realized it was a dream.

In the fourth I saw a tornado and recognized it as a dream sign because I had been doing a mantra about it, but still wasn't sure I was dreaming so I did an RC and bang!

In the fifth I was in a tornado but didn't recognize it as a dream sign. But I noticed an oddity in the physics as my house was spinning in the air and bang!

So now I do mantras and other prep work with the following dream signs:

Tornadoes
Friends with altered appearances
Odd sensations or attributes of my own body
Being in school (happens all the time in my dreams but I have yet to catch on in the dream state)

I could probably also use "naked in public". And I could even practice it by going out naked in public in waking life while doing reality checks!! "Don't worry, officer, I am just practicing my reality checks. Am I dreaming?"

I can tell you that in one of those dreams, even after I suspected I was in a dream I could NOT tell for sure because of the amazing vividness. Until I did the RC. So RCs are becoming very important. I practice RCs multiple times a day with sincerity. And I am able to get pretty sincere about it because I clearly remember NOT being able to distinguish the dream state without an RC. So as I sit here, I very well COULD be in a dream. RCs are my one thin tie to reality.