In "The Barbecue! Bible," Steven Raichlen recommends the "Mississippi test" for those who don't have grill thermometers.
Hold your hand about 6 inches above the coals and start counting "one Mississippi, two Mississippi," etc., until the force of the heat causes you to pull away. The number of seconds (Mississippis) you can hold your hand above the coals or gas-fired heat roughly corresponds to these temperatures:
High heat = 1 to 2 seconds (650 degrees)
Medium-high heat = 4 to 5 seconds (4oo to 450 degrees)
Medium heat = 6 to 7 seconds (350 degrees)
Medium-low heat = 8 to 10 seconds (300 degrees)
Low heat = 12 to 15 seconds (250 to 275 degrees)