Peter West Carey of DPS has a neat trick for always having a gray card “at hand”: he suggests using your hand as a gray card when you don’t have a card handy. You’ll need to start with an actual gray card for “calibration”:
In a nice even light, using spot metering and manual exposure mode, point your camera at the gray card. Set your ISO so it is not on Auto and maybe to 800, the number isn’t too important. Now adjust aperture and shutter speed until the camera metering is at zero, meaning it is not over or underexposed according to the camera. Next place your hand (I suggest your left hand) where the card was, with your fingers together. Ensure the center metering spot is completely covered by your hand.
What does the camera’s meter read now? Mine says the settings I had for the gray card are 2/3rds of a stop too dark for my hand. […] This means whenever I point the spot metering at my hand, and my hand is in the light hitting my subject, I just have to adjust my settings until my camera thinks the exposure is 2/3rds of a stop too dark and I am set!
So basically, since the color and tone of your palms don’t change very much, you can use the difference between your hand and 18% gray for snap exposure judgements while shooting.