Don't even look; just poke a finger through your palm. When dreaming, the rendering engine is still turned on, but clipping is off, and the physics engine is switched to a simplified model.

You can also breathe without an open airway, so the Valsalva maneuver does not work. As I need to do this several times during the day to relive underpressure due to my crap eustachian tubes, this is an immediate tell for me. Then I poke my finger through my palm, and usually get so excited about figuring out that I'm dreaming that I wake myself up. I've heard that you can spin around as though you were trying to make yourself dizzy to avoid waking, but it never seems to work for me.