Do the bucket test: Place a bucket a few inches into the water on a step or ladder in the pool. Add water to the bucket until it is even with the level of the pool water around it. Mark the pool water level on the outside and inside of the bucket. Secure the bucket on the step so it remains in the pool and stays the same temperature as the pool water. Wait a day or so. Compare the level of the pool to the level of the bucket. If the pool level is lower, you may have a leak.