Reddit mentions of BVV Standard Glycerine Filled Vacuum Gauge 0-(-30) inHg with a 1/4 Inch NPT Base

That should probably be fine. I might suggest a somewhat weak test of putting a slight kink in the hose pointing upwards... if the vacuum is struggling against it, it'll suck the hose back to straight.

Sadly, my testing methodology isn't particularly portable. I was given an 11 (?) inch absolute pressure gauge (Heise; this style) from the '60's, which I have up on display on the wall, but for fun is plumbed to a pneumatic quick connect. So I basically just patched it over to the vacuum using some pneumatic hose. A gauge like this could do the job, but TBH I don't think it's worth it unless you have another use for a vacuum gauge.

The connection part was stupid simple though: cut off a few inches of random 1/4" hard plastic tubing, drill 1/4" hole in the F-F coupling connector, jam the tubing in the hole, apply hot glue to prevent leaks. From there, it was just whatever connectors were handy. (I have a set of vacuum hold-down clamps, which helps explain why this array of stuff was readily available)

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As a significantly less scientific approach, you could connect a host straight to the vacuum, use a blast gate or even just a hand to block the flow varying amounts, and note how the sound changes. Then you can put the same hose setup as you normally use, just without the separator, and see how it sounds. Finally, with the whole setup, again. That should at least give a rough estimate for how much work and flow restriction it's all causing, and establish if it's due to the hoses, or due to the separator.

E: Addendum: that assumes that you have no leaks, and all airflow loss is due to pressure drop. If it doesn't sound like the vacuum is working any harder, but you get noticeably less airflow, then you likely are in the "leaking somewhere" regime. I don't think that's likely though, because you need a pretty enormous leak in order to appreciably affect the vacuuming performance of the Fein.