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.

Yes, my dreams are like… Low bandwidth while real life is like a lot of raw data. Often my dreams have a clear storyline or a punchline at the end. Also intuitive things work fine in dreams but non-intuitive ones does not like light switches wouldn’t work unless it is connected to the story or it has the main focus.

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

Because of the way that it is.