kbb.com will tell you what it's worth based on the condition (which can also give you a hint about how much work you might want to put into it). Before you trade it in, take it to carmax and see what they will give you for it, too. It can't hurt and if the dealer you are trying to trade it into offers less, you can take it back to carmax.
If you are up for it, you could also try selling it yourself. Small trucks like a Ford Ranger are pretty popular as a used vehicle. We were trying to sell our F-150 and people would stop by our house and ask us if we would sell our Ranger instead.
