How do you know when to fill the transmission fluid on a 1998 Chevy S10 Pickup?
When it is low. If it is indeed low, this means you have a leak. This information can be found in your owner's manual. If you do not have one then get one. Sources are, Dealer, Salvage Yard, E-Bay, and a Google Search. Type in "auto owner's manuals" and you will get several sites that sell them.