https://chapinmfg.com/products/chapin-6-8175-shut-off-assembly-poly-with-gauge

They do make pressure gauges for their shut-off valves. You may need to call/confirm that they fit your particular model though. The thing I have a hard time with is that they cost just about half as much as the entire sprayer does! I wouldn't buy one simply to check their advertised PSI - it's a simple calculation of how much water comes out a specific-sized orifice when a certain psi gets applied to it - but if you're wanting to get a feel for what 40 psi feels like and get your muscle memory built this could be a great tool.

 

What is posted above is a great way to get a real time check to the pressure in your tank. However, it won't actually do anything to keep the pressure constant as you're spraying (I think that's the ultimate goal, right?). The next thing I would look into would be a CF (Constant Flow) valve so that you can spray a consistent amount at a consistent pressure all the time. Your only job would then be to make sure there is enough pressure in the tank. From another thread where this was asked:

I would order a Chapin CF valve for your wand. They are available in 14.5, 21 & 29 psi ratings. A CF valve is designed to maintain constant pressure - so it won't start spraying until your backpack reaches the design pressure, then regulates down to that pressure no matter how hard you pump. I would probably choose the 29 psi CF valve for a backpack sprayer. If you wanted to create a little more deadband/operating range between the max system pressure and the point where it cuts out on the low end, you could step down to the 21 psi CF valve, but keep in mind lower pressures mean lower flow for any given nozzle. According to the charts, red -04 nozzles spray 0.40 gpm at 40 psi, 0.35 gpm at 30 psi, and 0.28 gpm at 20 psi. Droplet size also tends to increase as pressure is lowered.

 

Spot on. Thank you both, @quattljl and @corneliani .

@quattljl , what would be the best way to get a feel/muscle memory of a 40 psi or a 30 psi?

@corneliani , exactly my thought. I have actually ordered the Chapin 29 psi CF the other day. With that CF, I shouldn't worry too much now but I'd like to double-check things. These CF valves should be pretty accurate to their advertised PSI, like within +/- 1 to 2 psi accuracy?

 

@quattljl , what would be the best way to get a feel/muscle memory of a 40 psi or a 30 psi?

The only way to get a "feel" for it imo is by experience (and lots of it) with a calibrated sprayer. Anything else is just a guess. And then that "feel" really only applies to that particular setup because different tips have different flow characteristics, different sprayers might feel different in the hand or during application, etc. But even with that experience, you'll never be spraying at exactly 1 set pressure without a CF valve or even in the same ballpark without a gauge. You'll always be floating somewhere above or below the desired pressure, which will affect the output from your wand. With some of the high powered herbicides and things I spray, being "close enough", or "in the ballpark" just doesn't cut it, personally. I like to have as few variables as possible.