the synchronizer will let you adjust the airflow/throttle to get a precise intake vacuum balance.

Equal air/fuel mixture is not directly measurable (not easily). *if* your jets and needles are in good condition and perfectly centered, then the depth of the jet below the bridge of the carb should be the same and the amount of fuel metered by the piston/needle combo. Around 1/8" is supposed to be good, but it will vary significantly with carb condition.

You can check depth by removing the dome and piston/needle and measure the depth below the bridge that the jet is at. The tail-end of a slide-caliper is a good tool here.

Alternately...

All pistons being equal, the color of the sparkplug ends after a short trip should be identical (coffee-stained is what they call it).


Finally...
If you have a gas-spectum analyzer and want to shut of your carbs one at a time, you could measure the contents of your tailpipe gas...