If you suspect your fiberglass shower is leaking, it's important to make a positive determination as soon as possible. A leaking shower soaks the framing every time you use it, and the result may be rot, mildew and termite infestations. Water pooling around the base of the shower is one indication of a leak, and the water could be coming from the faucet, the drain or cracks in the walls or shower pan. A simple test will help you rule out the shower pan.
Stay out of the shower long enough for the pan to dry out, then remove the strainer, put a latex ballon in the drain and inflate it. Cover the drain with duct tape. Extend the cover several inches past the edges of the drain -- the taped area should be about the size of a dinner plate.
Fill a 5-gallon bucket about half full, using a faucet other than the shower faucet. If that faucet is leaking, water will appear under the shower and the test won't be conclusive.
Pour the water gently into the shower pan until the level is about an inch above the pan. If you suspect a leak in the curb where the pan meets the wall or the shower entrance, add more water to bring it up to that level.
Watch the floor around the pan for pooling water. If you have access to the floor underneath, check there for dripping water. Some leaks are very slow, so you need to check regularly for a period of about eight hours.
Add more water if the level in the shower goes down. This is a strong indication that there's a leak, but the test isn't conclusive until you see signs of water under the shower.
Remove the tape as soon as you see signs of water under the pan and let the water drain out. Refrain from using the shower until you replace the pan.