The odd thing is that despite reading stuff about reality tests and trying to build a habit from doing them while awake, on the rare occasions I've had a lucid dream I've just spontaneously become aware that I'm presently dreaming. I don't remember ever having a non-lucid dream where I've done a reality test.

Instead of fancy stuff like determining prime factors, one consistent dream sign I've had is utter incompetence in telling time from digital watches and clocks. This generally doesn't tip me off that I'm dreaming though, and doesn't occur often enough that I could effectively condition myself to recognize it.