Hooking up a pressure gauge can be pretty easy and cheap. First, find the hose off of the manifold that feeds your factory boost gauge. Then, go to an industrial or automotive supply retailer and get a 30psi pressure gauge, NPT brass fittings to fit to the gauge, hose barb fittings and a tee to tee into the existing gauge hose. As well, you will need an appropriate length of vacuum hose - 8ft should be more than enough. Total cost should be not much more than $30. Ensure that you check or test-fit all fittings with the gauge and hose before you start assembling. Tee into the existing gauge hose and attach the new hose, running it through a firewall grommet into the car. Affix the gauge to the hose and mount the gauge somewhere easy to see while driving. I drilled a hole into the trim panel below the steering column and ran the hose through, zip tying it under the trim piece for the heater fan control switch, since it was for temporary purposes.

This is what I did to measure the boost on my 90 745ti before I installed a G-Valve. The initial boost setting was 6psi at the manifold - my car has the (small) Mitsubishi turbo. At this level, I noticed a definite kick at full boost and the factory gauge would go to about 2 o'clock. With the G-Valve set at about 12psi, the gauge reaches around 5 o'clock.