How to tell if fuel pumps are running by current draw?

Ammeter shunts usually go on the negative, some absolutely have to be on the negative.

If you get a hall effect DC ammeter you can slide one positive wire through it and get a good idea of amperage flow, since running the ground through shunt might require one put shunt close to fuel tank. My fuel pump ground is nearby tank on the frame rail.

The hall effect sensors will not yield rock steady though, depending on RFI/magnetism in the area of the sensor.

I like the logic of your approach.

My dash gauges for fuel and coolant temp require 5v, and I replaced the old school mechanical regulator with a solid state sealed adjustable buck converter. Years later mid roat trip, I noticed the coolant temp gauge reading higher, but my K type thermocouples on a few key locations were reading as they always had and I knew the coolant gauge was reading incorrectly and I avoided a lot of anxiety.

Later, I put a 4 decimal place voltmeter on the output of dash regulator and saw it had drifted to 5.6553 volts. I reset it to 5.0002v and coolant temp gauge was where it should have been and removed voltmeter. A year later or so I noticed coolant gauge later drifted up again while Ktype thermocouples stayed as they always had, and I relocated the voltage bucker's potentiometer so I couldeasily reach it from driver's seat and reattached the voltmeter and was back up at 5.32v. I put it back to 5.000 and it has stayed there for a few months now.

I got one of these hall effect ammeters on my dash reading current into or out of my battery:
https://www.amazon.com/bayite-Digital-Current-Voltage-Transformer/dp/B01DDQM6Z4/ref=sr_1_7?keywords=bayite+ammeter&qid=1570775614&sr=8-7

I had it measuring total alternator output for a few months before moving the sensor to read amps into or out of the battery instead. It is not rock steady and will bounce upto 0.75 amps above or below actual worst case, but usually is about 0.3a + or - actual.

https://www.ebay.com/itm/DC300V-100A-200A-400A-Hall-Effect-Coulometer-Digital-Voltmeter-Ammeter-Sensor/223151298689?hash=item33f4da9881:m:mIvp1f5fhjq7eIfv_BWIFew

This one I have no experience with, but once you sort out your issues this can be relocated/repurposed to be more useful.

The inner diameter of the hall effect sensor is 3/4 inches so pretty big ring terminals on preterminated battery cables can be run through it, but not a post clamp.

Lots of Hall effect ammeters now available, the second link was the first available on a search, and the first link was just the product I've been using for a few years now, and have experience with.

Good luck Diagnosing the issue.