Start with KBB or NADA. I use NADA because that's the guide that every bank I've worked for uses. But these are just guidelines. They'll give you a range of values and you can expect the dealers to start at - or below - the low end. Ultimately, like anything else, it's worth what you can get someone to pay for it. If you're trading in, the dealers use an altogether different method of estimating value. They'll usually base it off what the vehicle is going for at the Manheim auctions or they'll call other dealers of the vehicle's marque.
Honestly, one of the best ways to get a good range is to scan Autotrader and see what people - especially dealers - are asking for vehicles similar to yours.