I thinnk I have a leak in my pond

I've had a pond in my back yard for some three years now, with pond
lilly's water iris and a few other plants growing around.

Problem is, this pond seems to be losing water at an abnormally rapid
rate, perhaps as fast as 2 inches in a week. Now I'm reasonably certain
it's not due to *evaporation*, the weather here is in the 30's and 40's,
dropping down in to the 20's at night.

The water is some 3 1/2 feet deep at the deeper end, so the fish
(Comets) are doing just fine, I've been running the waterfall, but I
guess I'll be turning it off now, it's about that time of year.

At any rate, I think I have a leak.. What do I look for to locate this
leak and then, how do I fix it?

Thanks, and happy Thanksgiving Day (or thursday if that's your fare)...