Does my pool have a leak?
The best way to determine if you are loosing water in your pool is a controlled water bucket test. Simply fill a bucket of water to the same level where your pool is currently at, place the bucket on the first step in the shallow end. After a few days, check to see if the water level in the pool has dropped below the water level in the bucket. The most common visible can leaks are found around the pool equipment, usually from a bad shaft seal in the motor, or from PVC pipe exiting the pump.
If you are loosing more than 2 inches a day it is safe to say that yes you have a leak, in most cases the root of the leak is minor and is a safe and quick fix.