Absolutely. Modern leak detection technology has made it possible to test even large water holding structures such as ponds and fountains. Not every company is equipped or experienced enough to handle large pond leak detection work, but there are companies out there that specialize in this kind of work.
Depending on the amount of water loss, the size of the recirculating system, and the condition of the pond interior, a good leak detection company should be able to give you a summary of all the work involved and a reasonable estimate for the leak detection.