Under good plumbing practices (This leaves out the helpers) you install a gauge before and after the regulator so you can actually see the pressure drop
Now if everyone thought like a helper you could imagine putting on a gauge on a hose bib connection which is a globe type of valve and offers lots of restriction and then having a home owner run like a mad man though the house turning on ONE faucet to see if it does cause any type of drop.
Great advice from a person who has no clue to plumbing piping size and or materials.
Suppose the piping is galvanized and had mineral deposits then watching a gauge will show you nothing.
Checking by the device is ALWAYS the proper thing to do.
hopefully the instaler did provide you with a by pass so you can repair this if needed without disruption in service Good luck