Your pool will naturally lose some water to evaporation, some to splashout and some to backwash wastewater. You may also gain water from rainfall. The rule of thumb is that if you're routinely adding more than two inch of water to your pool per week, you may have a leak. (that is worth spending some time and money to repair)

Pools are meant to be watertight, of course, but sealants will deteriorate, while other parts of your pool shift and settle, or just plain wear out. Pools can leak through any of the fittings or accessories, plumbing or even right through the shell. It is important to repair leaks, not only to save water, heat and chemicals, but also to prevent undermining pool structural components, and washing away fill dirt.

Leak detection is a highly specialized branch of the industry. 

 If you suspect a leak; look at the following things before calling for service:

• Is the pool leaking only with the equipment on? This may indicate a pressure side (return) leak. With the filter pump on, the plumbing on the pressure side is...under pressure. This can open up small drips into spraying gushers. Check the waste or backwash line for water running all the time. One inch of your pool water can equal 500 gallons.
• Is the pool leaking only with the equipment off? This usually indicates a suction side leak. With the filter pump on, the plumbing on the suction side is under vacuum; air can be drawn in through otherwise leaking voids. You may notice air in the pump basket (if you have a clear lid), air bubbling out of the return lines or air repeatedly built up inside the filter tank. Use tape or a pencil to mark water levels. Is the pump basket lid on tight with a good, lubed o-ring?
• Does the pool leak all the time? This does not rule out leaks in the plumbing, but turns a suspicious eye on the shell of the pool, looking for cracks in the plaster or tears in the vinyl. Look closely at the tile line, and look real closely inside of the skimmer's). The most common leak we fix is a separation between the plastic skimmer and the concrete pool. This is easily fixed with some pool putty. If you see something that looks like a crack, drop some of your pH indicator test reagent near it with pump shut off and water still. See if the dye is sucked into the crack. Under water lights can and do leak. Especially the conduit that runs from the light niche to the junction box. Filling the opening of the conduit, in the back of the light niche with putty, silicone or caulk is a way to fix this problem.
• Are there leaks at the equipment pad? Look closely at the filter, pump, heater and valves. Check the ground for moisture. Turn the pump on and off, looking closely for spraying water when the pump is turned off.
• Does the water seem to stabilize at any particular level? You may be able to close the skimmer valve and allow the water level to drop below the skimmer. If it keeps going, we can rule out the skimmer (although there can always be more than one leak). The underwater light is a common leak source. If the water stabilizes, dye test around this level very carefully. Look for small debris which may have been sucked into the crack or void. This is a good indication of a leak.
• Are there any wet areas around the pool? Take a walk around the pool's edge, and between the pool and the equipment pad. Check for wet soil and eroded areas.
• Is your pool a vinyl liner? If so, there are special considerations. Look for sinkholes where sand under the liner may have washed away. Look for tears or separations around all fittings: skimmer, returns, cleaner line, etc. Pay close attention to steps and corners, where the liner may be stretched more than normal. If an animal had the misfortune to fall in your pool, you may notice claw marks (tears) just below the water line. Spending time under water with a mask on may be required to find a small leak in the liner. When liners become old, they may have many pinhole leaks. There can always be more than one leak

Unsure of your evaporation rate? Place a bucket of water beside the pool and mark both. After 24 hours, check the loss of both. If the pool loses more, then there's a leak