At a meal break with the presses stopped, stand at the compressors for twenty minutes.
Count the minutes a compressor is pumping air, which its panel shows as loaded or as a percent of full output.
The presses draw no air then, so whatever a compressor pumps is going out through leaks.
Then write down the pressure on the panel and the highest pressure set at any press.
Cost
About $14: twenty minutes of one maintenance person at $40 per hour. Nothing is bought.
Duration
Twenty minutes, at a meal break
Confirmed if
A compressor is loaded for over about two of the twenty minutes, which puts leaks above 10 percent of its output.
Or the panel shows a pressure well above the highest set at any press.
Each result confirms one part of the problem by itself.
Ruled out if
Every compressor stays under about two loaded minutes of the twenty minutes, and the panel's pressure is within a few psi of the highest set at any press. This problem is then small here.