All ways an interesting one.
As stated above a sight glass is a tool and used as an aid to charging, generally speaking your pure refrigerants, are normally better than mixtures using a sightglass method. (with mixtures very small pressure drops, valves, driers etc. can cause bubbles) At times of very high load, it sometimes difficult to get a full sightglass when the valve is 100% open especially the TXV is slightly on the larger size of the required duty.
Superheat is also a good method, but normally only when your room or product (one pass chillers) is close to set point.
So it more down to understand at what stage your system is at and what condensing control you have. (also stated above)