The Ranger Station - Your Ultimate Ranger Resource
Look in the tech library for directions on checking with multimeter.
Checker Auto Parts, Schuck's Auto Supply, O'Reilly, Kragen Auto Parts and Murray's Discount Auto Stores, and most other auto parts stores have a cheap, (price) code reader, The one I got for my OBD1 cost $30 or less, comes with a manual that includes the codes.
If you are gonna keep the truck, spend the cash, it will save you $$ the first time you use it. Where I live, the mechanics charge $50 to pull the codes, then they won't tell what the code means.