Ok on my 95 Legacy L Wagon I have a R134a system.. from what I read you should not use the receiver/drier to check to see if the system needs to be charged?

 

Because what I notice is which the compressor turns on the sight glass has like a white froth that shows then it's back to clear with hardly any bubbles when the compressor is off.

When compressor isn't running, there's no flow through the lines. When it is running, some bubbles should be normal but a froth-like mixture might mean it's low on charge. You can't tell much without using gauges.

Using the sight glass on a system running R134a can be misleading. The info in this article might help: http://www.motor.com/magazine/pdfs/041999_07.pdf

 

R12 systems retro-fitted for R134a, depending on how carefully done and/or complete the conversion was, might not work efficiently.