Cc,
I was going off of your description that it was a "water table" pond. If you are not sure, then that question is a little bit difficult to determine, especially if you were not there for construction of the pond.
The pond level typically goes down in the summer for both sealed ponds AND water table ponds. I have two water table ponds that go down during the summer because the aquifer cycles fairly rapidly based on annual rainfall. Surprisingly, my water table ponds go back up in the late fall after the first hard freeze! It turns out the tallgrass prairie and trees in the area can actually "drink" more water than the flow rate of the aquifer. Once they go dormant, the groundwater level on my low ground actually goes up.
The water in a "sealed" pond is based on the rain and snow that falls into the watershed of the pond. The watershed is the land surface area that drains into the pond. Frequently, you can tell from the pond design and location if it was placed to capture run off.
However, groundwater ponds are also usually excavated in the low ground so the water elevation in the pond is closer to the surface of the land surrounding the pond. In that case, some surface water would usually also run off to the pond. If close to zero surface water would actually run into your pond, then you can be sure that you have a groundwater pond.
If not, it might be a little difficult for you to be certain of your pond type. The easiest way to test would be to actually pump water from another source into your pond. (Do you have another reachable source of water short of drilling a new well?) If you measured the water flow rate and the volume of the increase in the pond, then those two numbers should match very closely for a sealed pond.
If not, then observing your pond cycles would probably be your best option. A sealed pond should reach maximum water elevation within a few hours of a good rain. A groundwater pond may take a day or two to reach the maximum water elevation after a big rain. (Although it depends on your sub-soil geology.) Likewise, if you mark your water elevation and take readings to track the water loss, then you may be able to tell. A sealed pond should generally go down faster than a water table pond. Evaporation would be lowering a sealed pond by the full evaporation amount every day. Whereas the evaporative losses from a groundwater pond will be mostly made up from the water table. (You can put a pan of water in the shade near your pond and get a rough estimate of your evaporation rate.)
I hope those ideas help you a little to actually evaluate the true circumstances at your location.
If you do try some test pits or drill bores, you can sustain a groundwater pond from surprisingly thin layers of permeable streaks in your subsoils. I have dug some test trenches where I get about 2" of sand right on top of good blue clay. Those pits will fill up to the water table elevation in less than an hour. The same would hold true for a large pond.
