It's pretty simple. You can use a set of manometers or more likely vacuum gauges. Each inlet stub should have a port (usually blocked by a screw) to tap the gauge into. Then you can adjust the throttles so the gauges read the same at idle. If you have a good ear, you can do it with a hose, used like a stethoscope. You want to adjust the throttles so the sucking sound is the same across all carbs.
When I balance the twin carbs on my old Ducati, I pop the air cleaners off, back the idle screws right out, watch the front slide and stick my finger into the rear carb. With my other hand I just twist the throttle enough to make the carb slide move. When they're balanced, I can see the front one move, and feel the back one move as well, at exactly the same time. If they need adjusting, it's done by altering the throttle cable length where the cable enters the carb lid. Then I just need to adjust the idle screws to get a nice smooth idle and I'm done.
Hope that makes sense.

awww shucks.
It's kind of important having the carbs balanced nicely. Unbalanced carbs give the bottom end of the engine a hard time, can damage cranks and bearings. Especially old things with roller bottom ends and low oil pressure.

Very interesting, thanks for sharing folks!

Dobro Dan, Kirill, here is what you are looking for  



http://www.instructables.com/id/DIY-DualTriple-or-Quad-Carburetor-Sync-Tool/



Else, this is a more serious, "quasi-professionaol" tool for the "DIY expert"  



http://www.carbtune.com/