Fire Extinguishers can be a lifesaver in an emergency, but they can expire. Use Apartment Therapy's instructions to check your fire extinguisher monthly to make sure it's in full working order.
Make sure your extinguisher is in a place that makes it easily accessible. It makes sense to have more than one for different parts of the house.
As for the inspection, first do a visual check of your extinguisher. Make sure there are no dents or damage and that the hose is in good shape. Then check the nozzle to be sure the pin is in place and that the nozzle is clean and dry.
Check the pressure gauge. Is it in the green range? If not, get it serviced right away. The fire retardant chemicals in the extinguisher can settle to the bottom if left in one place too long. Flip the extinguisher over regularly to prevent this.
A fire extinguisher should last between 5 to 15 years, depending on the model. If you don't know how old it is or you think it might be time to have it refreshed, take it to the hardware store, who should be able to service your extinguisher and make sure it's in top shape again.