A trick that has worked for me: pull on a finger. In a dream (probably because I've conditioned myself) it will stretch like taffy which was enough to indicate that I'm dreaming.
Interestingly, after a while seeing my finger stretch wasn't enough to convince me that I was dreaming: I would question whether it was stretching enough. Like "oh, it's only double it's normal length, so this is probably reality." This has also happened with other tricks I've tried, like try to fly ("I'm only hovering; probably not a dream"). It's almost like you build a tolerance for particular abnormalities.
I always wake up when I do this because of the shock when I see how they look - sometimes like claws, or the bones are visible. Tried this as reality check to train lucid dreaming, because it's not really noticeable to others during the day - in contrast to the casual jump to check for flying skills
Mirrors don’t exist in my dreams, I don’t think.. I’ve never seen one that I’m aware of, anyway.. and I have a quite extensive lucid dreamscape.. interesting.
I find the easiest thing to do is check my pulse. I don’t replicate that in dreams, so I don’t feel anything where my heartbeat should be.
Only downside is I’ll never realize when I’m actually dead.