At a meal break, with the presses stopped, watch the compressors for twenty minutes.
Count the minutes a compressor pumps air, which its panel shows as loaded.
No press uses air then, so all of that air is leaking.
Then write down the pressure on its panel and the highest pressure set at any press.
Cost
Under $100: twenty minutes of one maintenance person's time. Nothing is bought.
Duration
Twenty minutes, at a meal break
Confirmed if
A compressor pumps for more than about two of the twenty minutes, so leaks take over 10 percent of its air.
Or its pressure is well above the highest set at any press.
Either one confirms its part of the problem.
Ruled out if
Each compressor pumps for under about two of the twenty minutes, and its pressure is within a few psi of the highest set at any press. Then this problem is small here.