I would want to test it dry....to be sure.I would think that it is thermostat controlled so water or air won't matter.
The one I have (different kind) the instructions said to put it in the freezer for a while then plug it in. The heater should come on for a couple of seconds then click off, this test is done out of water.
When I changed from the messy waterer you see in the farm stores to a nipple waterer I took the waterer out for a couple of hours then placed just the empty red bottom under the nipple. Within 5 minutes they all had it figure out. Most were pecking at the red bottom till one pecked at the shiny thing.
JT
Great nipple training tip!
Horizontal or vertical nipples?