Think about it for a second. The vacuum in each carb is made by air flowing through the the venturi section of the carb. The pressure drop (or vacuum, as it's referred to) happens as air flows through a section of necking diameter followed by expansion again. Speed increases in the smaller diameter section, and pressure drops because of that. It's the same theory that allows airplanes to fly.
Now that we know why it's caused, look at how air flows. Every time a valve opens and the piston goes down, air is drawn into the head, through the carbs.
So we can assume that the vacuum will be full of pulses, since it's being created by a piston moving air in strokes.
If you have a vacuum gauge, you should be able to hook it up and read this, although the pulses will be quick and likely make reading the gauge difficult. The other way you can validate this visually is by syncing your carbs using the two bottle method.
http://faq.ninja250.org/wiki/How_do_...-bottle_method Eventually, you'll see pulses of water vapor showing up in the bottles that corresponds to the speed of the engine firing.