One way to test it would be to take a 5 gallon bucket. Drill some 1/4" or larger holes in the bottom. Put an inch or so sand in the bottom, then pack 6"-8" of the red clay on top of that. Fill up the rest of the bucket with water and see how it holds.

The sand on the bottom isn't really needed, but I think it would give a better representation of what you're trying to accomplish when you're compacting the pond bottom.

If you wanted to do a couple of tests, try packing the clay in by hand and see how it works. Try the same test in another bucket but this time put 3" or 4" of clay in it packing tightly by hand, then take a basebal bat or something similar and pack it tightly by ramming the end of the bat into the clay. Pack another 3"-4" on top of the first layer and pack it again. Re-test. If there is a marked difference in the water loss rate, that would show you how different packing techniques change the end results. (i.e. sheepsfoot roller vs. packing with a bucket or just driving over the pond bottom)

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