If you cannot see any water, or get any to come out while inverted and operating the shaft, there may be very little water in there. I would not expect more than 1mL (20 drops) of water to be in there if you cannot get any to come out.
The capacity of the pump, itself, is pretty small, like 8-12 ounces. Most of that is the reservoir.
If you're set on treating it, you could fill the pump with (transmission) oil, and operate the shaft by hand, turning it 20-30 times.
But oil floats over water, so I would expect you'd want to invert it while turning the shaft. Empty the oil into a tub and let it settle. You should then be able to see if there is any water in the oil.
Mounting it into service, and operating it so it gets good and hot, will cook off any residual moisture that may be in there.