How are pond liners tested for watertightness?
Well, the proof is in the pudding. Usually the pond is just filled and observed to see if it is holding water. Close to 100% of the time it will be fine when properly installed. Very rarely there will be a problem. Humans beings what they are however, everyone makes mistakes. They are usually not a big problem to figure out, and are normally traced back to a minor inattention to detail around a pipe boot or something of that nature.