From a post to the Mr. Beer page on the Newbie forum:
If you don’t have a hydrometer, you can make a crude one using a shot glass and a Sharpie. Float the shot glass in half-gallon pitcher, see where the water is, and mark it at the water line. (If it doesn’t float properly, you can add “ballast” of a few nickels or quarters.) Fully dissolve a half-cup of sugar in the half-gallon of water, then float the shot glass again and mark the line. Do that with 1 cup, 1.5 cups, and then 2 cups. That gives you lines at 4 oz, 8 oz, 12 oz and 16 oz of sugar per half gallon.
Plain water is 1.000, your second line is about 1.023, your third line is about 1.046, your fourth line is about 1.069 and your fifth line is about 1.092 gravity. You can use a ruler to estimate gravities between those lines. (These numbers are if all the sugar is dissolved, and if you used the specified measurements.) There’s your own jury-rigged hydrometer! Cool, huh?
POSTSCRIPT: If you use ballast for your shotglass, you must ALWAYS use the SAME amount of ballast, for calibration and also for taking readings. The hydrometer works off of buoyancy, and since weight vs. displacement is buoyancy, varying the weight varies the buoyancy… so if you change up your ballast, your readings will be off.