The first stage of pool leak denial is attributing water loss to simply evaporation. Large differentials between water temperature and ambient air temperature, cross winds and pool water temperatures above 85 degrees F can all increase the rate at which water leaves the pool. Evaporation is generally very small if you keep a cover on your pool at night.
Pool Cover Evaporation
An easy test that you can try if you suspect that the water loss you are having is from evaporation is to cover the pool when not in use. Most pool owners have a solar bubble blanket however whether or not they actually use it is a different story completely. You can try making use of your pool cover when the pool is not in use and this would all but eliminate evaporation.
If the pool cover solves the water loss problems that you are having then great. If the pool continues to lose water then you can try one more test that will determine exactly how much water is evaporating and how much is leaking out via other means.
If you want to determine the difference between water evaporating and water leaking out from your swimming pool then all you need is a container that is see through like a larger plastic cup or clear plastic pitcher (a 5 gallon bucket works good also). You want something with a relatively wide mouth to give it as much surface area contact with the air as possible.
Sit the pitcher on your stairs or ladder tread and fill the pitcher to the exact same height as the water level in the pool. Be sure to fill the pitcher with pool water so that water temperature does not invalidate the test results. Leave the water in the pitcher for 24 hours and then compare the water level difference between the current pool water and the water in the pitcher. If the pool is lower then this could likely be a leak in the pool somewhere.
Water Evaporation Rate
From this test you can conclude if the water loss that you are having is exclusively from evaporation or not. If there is a marginal difference in the height of the water in the pool and in the container then do not worry too much. This test while useful is not 100% accurate as cross winds, sun exposure and other external factors can interfere with accurate results.
If you have any items like waterfalls, fountains or laminar jets in your pool this will also dramatically increase your evaporation rate in general so turn these off while you are conducting an evaporation test on your pool. During peak seasons (autumn) when evaporation is at its highest you could see as much as 1 inch per day in ideal circumstances evaporate from a pool. The average pool would actually see close to 1.5 inches per week of evaporation based on North American climate and median pool temperatures but one inch per day is not impossible.
Fail The Evaporation Test?
If you have discovered that your pool is in fact leaking then you will need to proceed with the next test which is another simple test that anyone can do. The isolation test will tell you whether the pool is leaking from the structure or from the plumbing system with relative certainty.