How do I know if it needs replaced?

What kind of tank is it? does it have a bladder in it? Does the pump cycle faster than usual? does you pressure drop very fast when you turn on a faucet? Does the tank leak? is it possible that the tank is waterlogged?

As long as it isn't leaking, or losing its air, no need to replace it. If it is over 30 years old, you might replace it before it does start to leak.

When they lose their air, the pump cycles on and off rapidly. You can replace the air by shutting the pump off and letting the tank drain. Then shut the water off to the house and add air to bring the pressure up to 2 psi below where the pump kicks in. Then restart the pump and open the valve to the house. For a long time, tanks have had a bladder in them to keep from losing their air. The bladder can fail.