Once the bathroom shower stall is framed in using plywood and 2x4s it's all sealed off with hot tar. Then a layer of water is floated in the shower pan over night. The construction guys will check in the morning to make sure the water level didn't go down, which would obviously indicate a leak, and need for a hot mop redo.
In this picture, the water is in the pan, and the drain is blocked off. It's difficult to see the water - it's clear after all, but it's there. You could tell that any leaks in the hot mopped tar are unlikely since the mop guy did a good job, applying a thick, even layer of tar to the wood shower framing.