[Coda]

It doesn't so much have to be water; any fluid that isn't also a diuretic counts.

If you pinch the skin on the back of your hand and it snaps back down as soon as you let go of it, that's a good sign you're probably sufficiently hydrated. If it doesn't bounce back right away (for instance if it takes a little bit to smooth back out) you're dehydrated.

(Edit: For people who don't know the term, "diuretic" means it makes you pee more than normal. Caffeine is one, although the concentrations in most teas and sodas isn't enough to cause a problem. Coffee's strong enough.)