I have a section on finding leaks at
http://www.fishpondinfo.com/pcare.htm#leakIt is geared towards liner ponds but most of it can work for a preformed pond. Most preformed ponds are less likely to have holes unless they are the thinner, less expensive sort. In general, you turn off the moving water and let the pond drain down to the hole. This may not be very good if either the hole is very slow in leaking, or it ends up being low enough in the pond to harm or strand the animals.