You should see some numbers stamped on the bottle/tag.... (2A-5BC) (2A-10BC) etc. Sizes are pretty standard and the small 5#-10# models are cheap enough that if your in doubt, replace it with a new one. If it's a dry powder, shake it upside down to free the powder, it tends to settle to the bottom specially shaking and bouncing in a trailer, if the powder is settled and packed down, the extinguisher will not work, no matter how much pressure there is in the bottle. If it has the plastic nozzle and valve, it's non-servicable. It's a one time use deal, use it once and throw it away. Check the gauge to see if it still has pressure. Check the date on the bottle to see when it was last serviced. And if your in any doubt, take it out back, pull the pin, discharge it and properly dispose the spent extinguisher.

Got a Lowes Home improvement near by???

http://www.lowes.com/pl__4294859071__s?Ntt=Fire+Extinguishers

Ace Hardware, Walmart, Home Depot, Internet stores, etc....

Maintenance tips....  http://www.firesystems.net/inspections/portable-fire-extinguishers.htm