I've never understood the purpose of syncronizing them. To me, if the butterflys are both closed when you tie the linkage between the two carbs together, then they are syncronized. I guess the action on the two individual pistons could be different, thus changing the amount of air and fuel being let into the engine.
If that's the case, then the best way to measure would probably be by using the venturi effect. A small 12 inch long piece of clear plastic tube with a ball bearing or BB Gun BB inside would suffice, I would think. Form a U-shape with the tubing, BB at the bottom of the U, and hold each one in front of the carb inlets. Whichever way the BB tends to move is the side drawing more air. If the BB stays at the bottom, the carbs are synced.
Although, now that I think about it, a BB could cause some damage if sucked in to engine, and something like gasoline or water might be a better measuring gauge.