When no water level shows on a sight glass, you can quickly tell whether it is completely full or completely empty.

Hold a pencil against the far side of the sight glass tube at an angle of approximately 45 degrees. If the image of the pencil viewed through the glass appears to run across the glass and changes little no matter what the angle of the pencil, the glass is full.

However, if the image viewed through the glass runs up and down the glass at a sharper angle than the actual angle of the pencil, the tube is empty.

Check this difference with normal water level before an emergency by viewing through the sight glass above and below the water line. Thick wall tubing gives a less pronounced difference, but a real difference, nevertheless. A little practice will take away any uncertainty.