Use the bucket test. Fill the pool to a normal level and fill a 5-gallon bucket the same level as the pool. Mark the water level on the inside of the bucket and the pool level on the outside (shut off the pump when you do it and then resume normal operation). After 24 hours compare the two. If the pool water mark on the outside of the bucket goes down more than the bucket water, there is probably a leak. If the levels changed the same amount it is evaporation.