if this a bladder type tank it will have an air valve similar to a car tire air valve. Remove the valve cap and depress the pin for a second or two to let some air out. If any WATER comes out or forms around the valve pin, the tank has a inner bladder leak. If no water comes out, then it can be assumed the tank bladder is ok. To put the tank pressure in correct working order, do the following:
shut the pump power OFF, drain the pressure tank of all water, then using a bicycle air pump, put air into tank so that the pressure in the tank is 2 lbs psi below the pumps "cut-in' setting. Turn the pump power back ON.
(on most pump gauges, the"on" setting is 30 psi and the off is 50 psi, then the air in the bladder should be 28 psi (when the tank is empty of all water) If it's a 20-40 setting then the bladder should be 18 psi
let us know how you make out.