You can measure water evaporation in your pool by doing the bucket test. Place a five gallon bucket on the second pool step and fill it to match the water level of the pool (probably have to put concrete block in bucket before you fill it with water to help keep in place). Over the course of 2-3 days the bucket will lose water from evaporation and gain water from rainfall at the same rate as your pool! As long as there is no splash-out or backwashing during that time, if the pool level drops more than the bucket level then you have a leak.