How do you know a fire extinguisher is ready to use?

Most extinguishers have a pressure gauge which has a Red Zone marked for full pressure. Just look to see if the needle indicated pressure at near the Full mark. Also, most extinguishers are filled with powder. This powder can pack down and cake when not agitated. It is a good idea to turn the extinguisher over, upside down several times. You will feel the powder move about inside the housing.