It's called a vacuum guage guys, and not that hard to read...

If you're pulling high vacuum at high RPM, you have a higher flow speed. If you change manifolds and pull less vacuum at the same RPM, then your intake velocity has slowed down. Of course, the best time to take these readings is at WOT starting from a fairly low RPM, and then wait for the reading to stabilize. If you read zero in/hg at WOT, then you're either at your optimum throttle body/runner size for your engine, or actually too large. If you're running more than zero [or actually less since we're dealing with vacuum] then you still have some restriction on the intake side. Also, at cruise speed you can use a vacuum guage to determine your most efficient throttle angle/cruise RPM [which I use on the tow vehicle].

Vacuum guages are available all over the place, and most boost guages have a vacuum reading on the low side of the scale, but of course people are only interested in the positive manifold pressure readings it can give.