you may not see a fall in boost pressure unless you are running near the maximum output of the turbo. I had an evo running 2.2 Bar (31.5 psi)

boost (and yes it kept breaking just about everything and was a race car) cost too much so now I have a 4wd.
I think like most of the others you have a cracked or leaking pressure pipe if you have access to a compressor you can pressurize the turbo intake to manifold and then listen for leaks (it is a bit of a bugger to setup but the only way we found the leaks in my car you can hear them). We used a car tire valve and sealed that into a metal tin lid and pressureised the whole thing just behind the air filter (stick the lid inside the pipe going to the air filter and put in 5 - 10 PSI with the throttle closed). if the crack is small it will squeal and if it is in plastic it will vibrate also, my intake was all stainless, it was always one of the hose clamps just finding the right one was the issue, if it holds (mostly you will get some leakage) the boost then you are OK.
Oh and you have to use launch control and antilag to move at the 2.2 bar boost pressures like I ran, nice on the drive line as first was boost limited to 1 bar (15 PSI), and as others have indicated turbo's themselves do not squeel without loosing a whole lot of oil into the intake or exhaust, this is where you can feel the movementon the turbo shaft.