A sight glass is fitted in the liquid line of systems with an expansion valve, usually after the receiver and filter drier. A clear, full glass normally means solid liquid is feeding the valve. Bubbles or a frothy flow can point to a shortage of refrigerant, a restriction before the glass, or simply a system still settling after start-up.
Many sight glasses include a moisture indicator, a coloured element that changes colour when the refrigerant is wet. This gives early warning that the drier is saturated or that moisture got in during service, before ice blocks the valve or acid damages the compressor.
Small capillary-tube cabinets usually do not have a sight glass, because they carry a small critical charge and are diagnosed with pressures, temperatures and a weighed charge instead. On larger display cases and remote systems, a look at the sight glass is part of every routine service visit, but it is read alongside gauge readings, never on its own.