A sight glass is commonly installed in the liquid line of systems that use a thermostatic expansion valve (TXV) metering device, and it allows technicians to view the condition of the refrigerant at the point where it is installed.

If you install it close to the outlet of the condenser or condensing unit, you can determine if the refrigerant is in a subcooled state leaving the condenser or condensing unit. Of course, it will not tell you the degree of subcooling, just if is subcooled or not. Installing it at this location might be useful while charging or checking the charge of a system, but again, since it does not tell you the degree of subcooling, it should not be used as the final determination of a properly charged system.