I've got one that's quick and invisible, but probably not all that failsafe. A while ago, I saw a RC method used by WILDers who didn't want their RC to accidentally bring themselves out of sleep paralysis. Try pushing your tongue through your front teeth. It's totally invisible, but if you're the type that has the finger-through-palm method fail on them, then it too is likely to fail.

Like all concrete reality checks, this should only be used to reinforce the "Am I dreaming?" question. The mental RC (and subsequent increase in awareness) is what you're really after.