I find that the best Reality Check is your own body. It's always there and you can't make up crap about why it's morphed! (like you can with the light switch RC that doesn't work - they mal function in real life along with other RCs that truly don't work - at least not as well as this one does.)

I would start out simple like counting fingers or memorizing where freckles or permanent things are like birth marks.

That and to become aware of your body movements/joints, touch (hot/cold, pain etc.) taste, sight, smell - but the best one is by looking down at your own body (sight). Counting fingers, noticing the location of things (such as your thumbs are in the middle when you see the tops of your hands.) Even looking at your fingernails, veins visible on your skin.

I would also recommed the Feldenkrais method which makes you super aware of your body. Most people aren't aware of the skin they walk in everyday!! People mostly focus their attention on external things, what they see, hear, etc - but they even zone out most sounds and feelings (like your watch on your hand). So as much as you can during the day, focus inward - it's sort of a paradox - like lucid dreaming is - you become aware of the inner and the outer all at the same time. Don't believe me that it's hard? Then try it, close your eyes and you'll realize you slouch too much or drag your feet, clench your shoulders or teeth and hold your stomach in.