First, fill the pool to the normal level.Fill a bucket, (an old chlorine bucket works well) to about three or four inches from the top, we want air movement over the bucket to be similar to that of the pool.Put a few bricks in the bucket to hold it down, and set the bucket on the first or second step of the pool. This will make the temperature as well as the air movement similar.Mark the water level in the bucket, and mark the level in the pool. (Usually marking a tile line with tape or using a sharpy on the inside of the skimmer works well.)Run the pool normally for 24 hours.Check both levels. The loss in the bucket should be very close to the loss in the pool. If the level has dropped more in the pool, you probably have a leak.Also remember, if it rains both vessels should gain the same amount of water, so a bit of rain won’t ruin the test.Sometimes you can get a bit more information with the bucket test.Set it up as before, but now run the pump 24 hours straight, then measure any loss. Reset the test, and leave the pool off 24 hours. If you lose more with the pump running, you may have a leak on one of the return lines.If you think you have an underground leak you will need the services of a leak detection company. These specialists can locate the leak using ultrasound and other tools, they can usually repair these leaks also.