If your pool is on the lawn, search around it for any patches of grass that appear to be greener than the surrounding grass. This can help you locate the general area of the leak. The water may be coming out in a great enough quantity to provide extra water for a particular patch on your lawn.
If your pool is on sand or dirt, check around the perimeter for any soaked areas. You may also notice a crater-like area where water from the leak has been collecting.
Wait until a quiet time of day, then use a rubber hose to listen for the leak in the area. Hopefully the previous two steps have helped you narrow down its probable location. Listening can be accomplished by filling the hose with water, plugging both ends, then putting one end against your ear. Use the other end to probe the liner in the area where you suspect the leak is located. Even a small leak often makes a detectable sound.
If the sound has gotten you closer to the leak but still has not revealed its exact location, swim to the bottom of the pool with a bottle of brightly hued food coloring. Squeeze out a small amount in the suspect area. If you are close enough, the dye should be drawn toward the hole in the liner. You can also buy special products that contain dye made especially for pool leak detection.
If you are unable to pinpoint the exact location of the leak. add a leak seal product to the pool water. Since the leak is too small to be easily found, it is probably small enough to be sealed by the product.