Perform WBTB. That is, go to sleep and wake up a few hours later.
Gently move your fingers as if you're pressing two piano keys back and forth, but without actually moving them.
After a few seconds (or, if you know you take a while to fall asleep, a few minutes), perform a reality check of some kind, preferably a motionless one. False awakenings are reported to happen often with this technique (and others), so make sure you double-check before assuming anything.
Hargarts' FILD post on DreamViews