You can get a flow meter, you need 1 specifically for the carbs you have.  They have different rubber adapters for different carb barrels. More than anything you need to have QUALITY linkage. Cheap or worn out linkage may as well not even be on the carbs..they will cause more problems and WILL be impossible to set up. The most recent example I have seen was junk linkage that worked until you stomped on the gas and then it would either bind up and stay at WOT or 1 of the linkage arms would simply fall off.  Can make good carbs worthless...far easier to deal with a single carb on a daily ride.  You can use a rubber hose to listen to the pulse in the venturies and adjust until they sound the same, no as precise as a numbered flow meter but it does work with some practice. Still though, linkage is where it's at.