How can I tell if the bladder in my water tank is damaged?
If you measure 0 psi when the tank is empty, it means the tank is not charged. Try adding air through the Schrader valve. If you can't get the pressure to increase and hold, the bladder is bad. 

If you measure the pressure at the Schrader valve when the tank is connected to the plumbing, you'll always measure the water pressure in the plumbing. The tank must be isolated from the plumbing, to measure the air charge pressure.