For me, the best unobtrusive reality-check, by far, has been checking gravity. Just by quickly changing my weight in a chair or rolling up a bit on the balls of my feet, I can see if my weight is there or I start to float a bit.
Checking global light level control (light switches) is, like you say, not unobtrusive. But I've tested, with moderate success, using more subtle lighting features. Window blinds, for example, typically have far more subtle shades of light and shadowing than gets rendered in my dream worlds. Paying a few seconds of focused attention to a window or lamp shade doesn't stand out in most social situations.
Checking global light level control (light switches) is, like you say, not unobtrusive. But I've tested, with moderate success, using more subtle lighting features. Window blinds, for example, typically have far more subtle shades of light and shadowing than gets rendered in my dream worlds. Paying a few seconds of focused attention to a window or lamp shade doesn't stand out in most social situations.