The red float will float up and tell you that you have water in there.
Checking Water Separator
1. As water is collected in the water separator, the red
float is raised.
2. When the red float has reached the white line, close
the fuel cock, loosen the retainer ring, take out the cup,
and clean the cup. Be careful not to break the element.
3. Place the cup back into position. Bleed the fuel
system.
(See "SERVICE AS REQUIRED" in "PERIODIC
SERVICE" section.)