Not sure what size your pond is but by the looks of it... if you see more than 1/8 inch a day during winter you may have a leak or some seepage. Of course this estimate would vary depending on the weather. Would be much higher in the summer.
The best way to tell (other than visible signs of water leaking) would be to measure the water level daily and track it as it fills. Then you will be able to see major changes. You would also be able to see the affect that rain has on it.
I'm in the same boat. Repaired my pond last year and I'm still trying to determine if I still have a leak. I've found that tracking the water level will give you a pretty good idea of what is going on.
