Sight glass

A viewing glass or window set in the refrigerant pipe of a car's air-conditioning system, usually in the top of the receiver-drier, which allows a visual check of the refrigerant passing from the receiver to the evaporator. The fluid should be clear and free from foam
A glass window in the liquid line, the top of the receiver-drier, or a test manifold, for checking refrigerant flow.
Glass tube or glass window in refrigerating mechanism. It shows amount of refrigerant or oil in system and indicates presence of gas bubbles in liquid line.