well if visual inspection isn't working and you can't hear any hissing from vacuum leaks a pressure tester is a must have for turbo owners.

just google "boost leak tester" and you'll see what I"m talking about. I do recommend one with a gauge because with a boost leak tester, you pressure the entire engine, even bottom end if your car has a pcv system that ties back into the intake. you'll want one with a gauge so you don't over pressurize the system. I recommend no more than 10 psi for a pressure test even if you run higher boost pressures than that you have to remember the boost leak test will put pressure every where on gaskets and the back of seals. it will not hurt anything as long as you don't over charge the system.

so once you have your boost leak tester, you remove the coupler on the MAF and clamp the tester in it's place. use compressed air to pump up the system and listen for air leaks. and repair, and retest. and that's it.

If you own a turbo car you should own a boost leak tester. I can't tell you how valuable mine is to me

hope this has been helpful