To determine if low water levels in your pool are the result of evaporation or a leak, you can employ the simple bucket test. 

1. Fill a plastic bucket three-quarters full of water. 

2. Mark the water level of the water on the inside of the bucket. 

3. Put the bucket in the pool, 

4. Mark the pools water line on the outside of the bucket. (If it bucket has a handle take it off to allow for better stability while floating. 

5. leave it there floating for two or three days. 

6. Inspect

If the water inside and outside the bucket has gone down the same amount, the pool is losing water due to evaporation only. However, if the pool water level has gone down more than the water inside the bucket, it has a leak. It which point there may be professional attention required.