How can you tell if your water level dropping is normal or a leak?

Fill a bucket with water, then mark the water level on the bucket with a waterproof marker. Set the bucket next to the pool. Next, mark the water level of the pool. After a couple of days, check the marks on the bucket and the pool. They should measure the same (having lost the same amount of water due to evaporation). If the pool level has dropped more than the level of water in the bucket, you have a leak somewhere. (By the way, make sure your dog hasn't drank out of the bucket, or pool!)