To check the air pressure in a bladder tank the power to the pump must be turned off. Then a faucet should be opened until water stops coming out of the tap. While the pump is off and the faucet is still open, you can check the air pressure at the schrader valve on top of the tank. You will need to use a car tire pressure gauge on the schrader valve. The air pressure in the tank should be 2 to 10 PSI below the "start" pressure of the pump. IE; with a 40/60 pressure switch, the air pre-charge in the tank should be no higher than 38 PSI and no lower than 30 PSI. Use an air compressor to pre-charge the tank to the correct pressure. If the air is coming back out of the open faucet, then the bladder is busted. If while holding the valve core down on the schrader valve, water comes out, the bladder is busted. If the schrader valve will not accept air from the compressor, the bladder is most likely busted.
From the sticky above titled "Pressure tank maintenance".