Just count your fingers, if you’re dreaming there will be way more than 10 of them

For me it’s two things:

1. In a dream I seem to have some ability to force myself to “wake up” if I really want to. I haven’t seriously believed I could do that when I’m awake, but who knows - maybe being awake is just a much more convincing dream.
2. Sometimes when I’m dreaming there are weird enough things happening that I do suspect that I’m dreaming.

Because of the way that it is.