Sight Glasses are used to determine the state of charge. 

For optimum performance a cooling system must have a solid column of light at the expansion valve. 

A sight glass provides a visual indicator of any "bubbles" in the refrigerant stream that could adversely effect performance. 

In general, the sight glass should be located as close to the expansion valve as possible. 

Sight glasses also incorporate a visual moisture indicator which changes color if moisture is rises to an unacceptably high level.