Looking at the bottom picture, take that fuel pressure gauge fitting off the feed line to the rear carb and put it in the line coming in to the regulator. Which looks like it’s the line right above it.
That will tell you how much pressure you have getting to the regulator.
EDIT: I would eventually move that little gauge over to the pressure side of the regulator so any time you want you can see what the pressure is getting to the regulator.
You don’t need to monitor the pressure to each carb inlet. They will both be the same.
