I made a gauge from wine bottles. Takes three bottles with corks and tubing. Drill holes in the cork tops for the lines. I used the plastic tubing used for refrigerator ice maker lines, but then you need some rubber hose to connect those lines to the carbs. Put the 3 bottles in a line. 2 lines(one from each carb to one outside bottle) come from the carbs go to the outside bottles and the lines stop at the bottom of the corks. then one line goes from the bottom of each outside bottle to the bottom of the inside bottle. The inside bottle gets 1/2 filled with water. When you hook them up, the unbalenced carbs will suck the water to one side, but since the line to the carb is at the bottom of the cork, and only half filled with water, you don't suck water in the carb. Of course, its balanced when the water will stay in the middle bottle. Use the carb adjustment to move the water back in the middle bottle when it gets sucked to one side. Its very accurate, and sometimes hard to get it completely balanced. I finally got a gauge, but this works fine and its cheap. If you aren't a wino, any bottles or jars work fine.
Somewhere on the web I'm sure there are pics of this, I didn't have time to link.