Place a bucket of water beside the pool and mark the water in both pool and bucket. Wait for 24 hours and then check how much water is lost in both. If the loss is greater in pool, then you have a leak.
Place close attention to the water you are adding:
If you are adding water to your pool more than once, then you have a leak
Area around the pool:
If the area around the pool is always wet, then you have a leak