If you have an inground swimming pool system that has an air leak and the pressure is 12 psi with no water leaks how can you find out where the air is coming from?

Do you have air bubbles coming back to the pool thru the return lines? If so, you have a suction leak. Could be between the skimmer, maindrain, at the pool pump. Most likely place to start is the pump. Check the lid o-ring for proper seat and fit. Check the pump seal. Check any fitting that goes into the pump pot - in and out plumbing plus small plugs at the base of the pump.