To find a leak in a pond, fill to normal water level, mark the water level, and turn off the pump. Then leave it alone for 12-24 hours and check the level again. If the water level has dropped, the leak is in the main basin of the pond--most likely there is an issue with the pond liner. Move any living creatures to a temporary basin, and continue to take regular readings. When the water levels stop getting lower, you’ve found the depth where the leak is, and need to carefully examine the liner around its circumference till you find the issue. The easiest and most likely problem is that the liner has been moved out of place.
If the water level doesn’t recede with the pump turned off, then you have an issue with the plumbing.
Note: If you have a waterfall with either a PVC check valve or a waterfall filter box that receives underground plumbing directly into it, start by inspecting the check valve and the pump.