Usually you turn off any pumps for a bit and see if it's still leaking. If it is still leaking then you let it leak until it stops. Your leak is somewhere at that level.
If pumps off stops the leak, then check the plumbing, streams, waterfalls, etc. Bypass parts of the system if you can to try to isolate where the leak is coming from.
Look for wet spots around the edges, plants or debris half in/half out of the pond, etc.
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