A manometer is a device designed to measure pressure. You can buy them but its just as easy and dirt cheap to make one. The concept is basically like drinking through a straw - you suck on it and the liquid comes up the tube. In this case, its the suction in the airbox (or wherever) thats causing the fluid to rise. There's a formula to work it out, but every 1cm the fluid rises equates to 0.014psi (at sea level anyway). Pressure in your intake (prior to the throttle body) would be roughly atmospheric (approx 14psi) at idle. When you rev it up though, the air can't get through the ducting as quick as the engine could use it - so its restrictive. If the ducting were perfect (leaving out systems like on the EF onwards for the moment) the pressure at the valve would be the same as the pressure at the mouth of the intake for maximum volumetric efficiency. As I've found however, there's about 0.6psi drop even before the throttle body, reducing airflow and therefore power potential. As it stands, the stock setup is giving away somewhere in the region of 6kw, and more if you've increased the airflow (ie;power potential) of your engine. I'm not sure why the EA-ED's have such a small intake snorkel. It might have been to cut down induction noise (the XH snorkel in my car makes a deeper sound that sets off this annoying buzz in the dash that I can't find) but it could equally have been an attempt to keep gas speed up as an aid to bottom end torque. If you've ever driven your car without any of the intake ducting (throttle body open to the air-not really a great idea) this is why the thing actually feels slower - low gas speed through the manifold.
Progress is like a Commodore, if you don't keep pushing it, it stops.