Wipe the floor of the shower pan with a towel to remove all moisture, especially around the drain opening.
Lay strips of duct tape over the opening and extend them out at least 6 inches on either side of the drain. Overlap the pieces of duct tape until you have completely covered the drain hole. Press down very firmly around the perimeter of the duct tape to ensure it forms a watertight seal with the shower pan.
Place a tape measure at the back of the shower pan where it rises a few inches up the wall. Measure 1 inch from the floor and draw a line on the shower pan wall using a dry erase marker.
Walk to another water faucet in your home and fill a large bucket 3/4 full of water. Return to the shower pan.
Pour the water into the shower pan slowly until it reaches the level of the marker line.
Let the water sit in the pan for up to eight hours. Check the pan every 30 minutes to one hour to see if any water has leaked out onto the floor surrounding the shower. Also examine the water level. If it has sunk below the marker line, the pan is leaking.