Sight glass is useful for an R12 system - I don't even hook the gauges up, just top off until the "foamy" appearance is minimized (usually about a pound for my 300E).

For a R134A system, you really need to hook the gauges up and check the pressures for correct charge. My 380SE is a converted system and still has the sight glass. With perfect charge, it still looks "foamy" in the sight glass. If I kept adding refrigerant until the foam went away, the system would blow up!

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280