One of the most important things is to know the age of your water heater. You can find the age of your water heater by looking at the manufacturer sticker on the upper portion of the water heater. It will have the brand name and many different numbers and small symbols. Try to find the serial number or manufacturer date.
This date will not be exactly when your water heater was installed, but it will tell you how old the tank is. The serial number will have a date code for example "F051052638". F is for the month - F is the sixth letter so it represents the sixth month, June. Next the first two digits of the serial number are 05, this represents the year, 2005. A water heater with the serial number F05052638 was made in June 2005. Each manufacturer has a similar "code date" for their product, some are easier than others to figure out, if you can not figure out the age of your water check the manufacturers website for the way to crack their "code".