+1 for a liner. Be sure to use one designed for fish in ponds, not roofing rubber, etc.
But if you want to find a leak...
Let the water leak out until the water stops leaving. At that point, yiou can assume that the leak is right at the waterline, somewhere. Now that you have narrowed the field of possibilities, get a dropper and a cup of milk. Raise the level of the water in the pond by 1 or 2 inches, get down close to the waterline, and place a drop of milk in the water near the edge. Watch to see if it goes anywhere that it should not. If you do not see anything indicating a leak, move a few inches away from that spot and try again. This is slow work, but if you take your time, you may very well find the problem.
Last edited by glassbird; 01-08-2011 at 08:40 PM.