The air assist is critical for keeping flare-ups at bay, mitigating smoke, helping keep the lens cleaner and cooler (it really shouldn't get that hot if it is in proper order anyway), and, when cutting, catalyzing the ablation of material and evacuating the debris field through the bottom of the substrate and out the downdraft exhaust.



Here is more on external air: Thunder Laser External Air Compressor Guidelines

The maximum recommended operating system pressure, according to Thunder China Engineering, is now 40 psi to help prevent premature air solenoid failures.

Other than sound, feel, and maybe little bit of napkin math, there is no real way to apply a metric without a gauge. You can monitor the pressure at any point in the air assist system... the question is, which point?

Well, the placement of the peripherials, as well as the components and methodologies used, can make pressures vary from user to user unless they were modified in exactly the same manner with exactly the same parts. But, at the end of the day as long as you set a constant reading, you will be able to replicate those outputs. 

To give you an example of how sensitive pneumatics can be, here is some information that Chris Ankrom published regarding the pressure difference between a T and a Y quick release coupling:

If installing a pressure gauge for air at your laser head, installing a T verse a Y makes a pretty big difference. 1st pic shows gauge reading with T installed.  4th pic shows reading with Y installed. Last pic is pressure coming to laser. Noticed around 6 plus psi different with Y verse T.  The Y seems to be more accurate to my input air.  No picture posted for test but also tried rotating the T so that air from the air assist dials was connected to the leg of T and it had no change.
The installed T
 Reading with T installed

The Installed Y
 Reading with Y installed

Thanks for the great observation Chris!

Here is a dual gauge system:

Gary Wiant measures at each stage and outlines his setup and parts list here This is a pretty aggressive and involved install:

Here is his part list:

Here is a less invasive and complex way:

 I chose to monitor the line going to the nozzle upstream of everything else because that is where the reading will be close the nozzle pressure. I am using an SMC ISE30A digital pressure switch that was used on the Mars series lasers:

Here is the digital pressure gauge mounted and displaying psi. i can set a lower limit and the display goes red if that limit is reached. it can also trigger an output when in alarm state. I am not going to signal the dsp to interrupt for low air assist pressures air but it is an option. Here is the link: