Low oil pressure follow up

Well last night on way home it dropped to the line right at the edge of low and it didn't leave there most of the way home. It has proper oil level. After it had cooled I started it again and now it starts at only half way on gauge instead of at 3/4. It is getting worse, but it still hasn't dropped low enough to have any bells or light for warning of low oil pressure. I have been tol oil pressure sensor, oil pump, leak which it is not loosing any oil, and oil screen sludge or wore internal parts. How can I find out for sure which is the actual cause. I know the sensor can be tested with a pressure test and the screen, the pan can be dropped, but how do I know if it is the oil pump or wore out internal parts without tearing the whole motor down?