My understanding is that if you can easily remove it with your hand then you are OK. It is when it is very very difficult to remove by hand that you have a problem. Once it is off (even with a normal AOS) you will hear a lot of swooshing type sounds.
The car will start to run poorly because of the large vacuum leak that just happened when you took the cap off.
There are a couple of threads in here about measurement methods of a vacuum that you could also use. Search for "slack tube manometer", here is one thread:
http://986forum.com/forums/general-discussions/45878-measuring-crankcase-vacuum.html
Steve
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