Do you remember as a kid, blowing bubbles with a straw in a glass of soda or water? Well, if the water in the glass was one foot high, it would take exactly 0.433 lbs of pressure before the bubbles would be ejected from the bottom of the straw. If you place a sturdy tube down to the bottom, possibly copper, then run a poly tube to wherever inside your house. Inside the house, you will place a 'T' and a schader tire nipple. This you will attach to a hand tire pump. The other side of the 'T' is attached to a pressure gauge like 0-5 psi. If the tank is 8 ft. high and is full, the pressure gauge will read: 8x0.433=3.5 lbs. What you do is gradually pump the poly line until the pressure no longer increases. This tells you that air is escaping the down pipe inside your water tank. If you can't find a low pressure gauge, you can fabricate one out of poly tubing with a small resourvor on the bottom of the verticle tube, putting in some coloring to make the water level more visible.
Gene