An established reality check is the nose-check test (You check to see if you can see your nose and if it's not there you are dreaming). I've developed a variation of this that may be even more discreet:
I observed I can actually see my lips sometimes when I talk and say certain sounds and this may be a good reality check. Building on this, I worked out an RC that can be used even when I'm not talking. Here's how you'd do it:
1) Keep your mouth shut but fill your mouth with air (like when a trumpet player puffs their cheeks), then exhale the air. When you are awake, your lips are visible, when you are dreaming, the lips cannot be seen.
As simple as that. This RC can be "disguised" as an exhalation - or sigh - the sort that you do when exasperated. It can be done in a spit second by filling your mouth with air but immediately - at exactly the same time - doing the exhalation through your lips. It's all over within about a second.