Old or new.. Get one of the following
A pressure gauge, if you can not find an "Inline" model then add a "Y" connector This goes on the OUTLET side of the next item:
Pressure Gauge w/Garden Hose Adaptor - FreshWaterSystems.com
I actually do not like WATTS gauges all that much.
Watts, 1/2 or 3/4 inch adjustable regulator with hose adapters or a
Zurin 1/2 or 3/4 inch adjustable regulator with hose adapters.
WATTS Pressure Regulator, 3/4 In, 10 to 25 psi - G5308773 at Zoro
NOTE: links in this post are EXAMPLE only so you know what to look for..I shop local plumbing supply, Lowes, HomeDepot,Mennards for these products.. Though you may find the odd CW that has a bag with all the needed stuff in it.
OR; (Skip the pressure gauge if you get this one:
Amazon.com: Valterra A01-1117VP Brass Lead-Free Adjustable Water Regulator: Automotive
Amazon listing,if viewing long time from now may have expiered
I have used Watts and Zurin, when new they are fantastic, When aged you start to loose pressure, but rebuild kits exist.