i may be coming out of left field here, but you cant know if a pond is sealed until it has water in it. all you can do is construct the pond with best management practices/techniques/resources available within your budget and be confident that you've done what it takes for the pond to hold water.
in our climate, if you let a pond dry, the shrinkage and swelling of pond bottom clay can produce fractures that will not heal, and will lose water. these fractures would need to be pulverized and re-compacted prior to re-filling.
