You need a special tool called a synchrometer. Inglese sells them for $65 US. I think I got mine from Mike Forte, but I don't remember what I paid for it. Here's a picture of one, with a cheap price:
ACC-C10-7125 - DELUXE CARBURETOR SYNCHROMETER TOOL They can only be used with an air horn.
You could make a manometer like Gav said. But you have to be very carefull about how you use it. If you just wave the hose end around the throat, you'll get a variety of readings. Not very accurate. Maybe if you taped it the inside of one of the horns, and then move the horn around. Then you'de get repeatable result.