2 tests:
Pour a number of buckets of water directly down the drain and then check for dampness. This will test the piping without getting the pan wet.
I would wait several days to be sure the area is dry, then place one of those large rubber "disc" drain stoppers over the drain. Run the shower long enough to flood the pan to a depth of maybe 1" or so. Let sit for several hours and check for dampness.
Jim