I have seen those sight glasses in the gauge set. It looks like it visualizes the flow quality of the CHARGE line. How could you get the sight glass to visualize the liquid line flow? You would have to get the flow going from the high side hose to the low side hose i.e. in series with the liquid line.
In my research I have come to realize that the receiver dryer is being dropped: the charge amounts are dropping significantly;; and that slugging of the compressor is more of risk. If the system runs low on refrigerant the oil collects in the evaporator. If the system is overfilled with refrigerant or oil, the compressor is at risk for slugging.
Gone are the days of throwing a can of refrigerant and a can of oil into a system and calling it good.