1. Age
Tank water heaters commonly last about 8 to 12 years; tankless units often last longer. The manufacture date is usually in the serial number on the label.
2. Water around the base
Check the fittings and the relief valve pipe first. If water is coming from the tank itself, the tank is corroding from the inside and can't be repaired.
3. Rusty or discolored hot water
If only the hot water is rusty, the tank or its anode rod may be corroding. If both hot and cold are discolored, the problem may be your pipes or water supply instead.
4. Rumbling, popping or crackling
Sediment builds up on the bottom of the tank and makes noise as water heats through it. Flushing can help on a younger tank; on an older one it's often a sign of wear.
5. Less hot water than before
Running out faster can mean sediment is taking up space, a heating element has failed on an electric heater or a part on a gas heater is failing.
6 and 7. No heat, or water that's too hot
No hot water can be a tripped breaker, a pilot that won't stay lit or a failed part. Water that's scalding or a relief valve that keeps releasing is a safety issue: turn the heater off and call a plumber.