This will give you an idea
http://www.kbb.com/
But, the only way you'll really know is to drive it onto some used car lots and ask what they'll pay you for this truck. It doesn't make any difference whether or not you're trading for a new vehicle...the wholesale cost to the dealer is always the same--although he might tell you differently if he's trying to flimflam you during the deal.
Also try looking into your biggest regional sunday paper classifieds...call the folks and ask if their truck has sold at the price they're asking.
Ken