Vacuum leaks are another story surely? Generally speaking I will apply a vacuum (or try to) with my Mitty vac & observe that the negative pressure is maintained. Or the other method would be to observe the vacuum that the system generates & compare it to an expected value. As an example, the vacuum developement on many modern Turbo Diesel cars for turbo & EGR control is via the brake servo pump & I expect to see approx 20 inches of mercury develpoped within a second or two.
Another good clue of course is a brake pedal that only has one good application before the vacuum depletes & you nearly run into a wall!
I agree with your comment about smoke testing. It can flush out many small leaks that could take hours to find by otther means. I have been slighty thrown on a couple of occasions though. A Saab commin rail diesel a while back had several leaks in the intake, at around the swirl flap spindles & I was close to ordering the parts. Something didnt feel right & I carried on testing. I found the EGR stuck wide open & followed that route with a successfull fix.
Although I don't have a Smoke-Pro, I have mackled myself a version that works very well. With a Disco smoke generator & a E46 secondary air pump & a Transit heater fan resistor to control flow & pressure. Total cost of approx £100 I am happy with it.