Every water heater has a label, and labels can tell you all kinds of useful things. Take a look at the residential water heater label at right. The date of manufacture -- one of the most important bits of information -- is included in the serial number. The leading digits show this tank was made in January 1997 (the 01 97 is for month and year).

It has an insulation rating of R-6.3. It's made by Rheem of Alabama. It is model 21X40-7N. Its capacity is 40 gallons and its Btu input rating is 34,000. Btu rating is more important than total gallonage in figuring how powerful a water heater is.

Commercial water heater labels follow a similar scheme. You can tell the age of most tanks (although there ARE exceptions to this) just by looking for something that resembles a month/year or week/year in the serial number.