Each extinguisher comes with directions specific to their product. Generally speaking, the extinguishers should be mounted in a readily accessible location away from a potential source of fire. Review the instructions. An extinguisher will be no good to you if you don't know how to use it. Check the pressure in the extinguisher.
Most extinguishers have a gauge or other indicating device.
If the pressure has leaked out it will not work. Check the nozzle for obstructions such as wasp nests, gum, spider nests, dust, etc. The nozzle should be free of powder too. If you see dry chemical residue, not dust, it means the unit may have been shot and may not be any good. Make sure the unit will come free from its storage bracket easily.