I was at the local supply house asking this question last week.

The guy is generally well informed and had just been to a class
on 407C. He said the two methods are to cut out the compressor
and tip it, measure it, etc.. The other is to use this special pump
they sell. It looks like this:
http://www.rsd.net/search/item.php?i...ump&searchtab=

With the pump, he said you have to cut the little nipple on the compressor. (I don't typically
examine compressors that closely, but I'm not sure if they all have such a nipple. I know
my mini-fridge's compressor has one because I see it all the time, but I'm not sure I've
seen them on typical a/c compressors. I'm sure others can clear that point up.)