How many gallons of water should a swimming pool lose daily?

That all depends on the size of the surface of the pool the amount of people going in and out of the pool etc. To see if a pool is leaking take a bucket of water out of the pool, mark the water level of the pool and the water level in the bucket. Then leave the pool and bucket alone for a day or two. if the water level in the pool has gone down more then the water level in the bucket then there is a leak or water is being removed in some way. If the levels are the same then the water lost is a result evaporation.