Most in this area use black book (black book.com). Many local dealer web sites have a link labeled something like "value your trade". But the links invariably take you to black book.
When I bought my 2012 F350 I did go to that site and got the black book trade in value they posted. It returned a value of something over $16,000 at the low end to nearly $$22,000 at the high end. Dealer advertised $14,000 off sticker (end of model year rebates were good) for his and offered $17,000 for mine. When I showed him the value of up to nearly $22,000 from his own link to black book and said I wanted that for mine he bumped it to $18,000. I started for the door. He bumped it to $19,500. I started for the door again. He offered $21,000. Needing to be some where soon and that near the high I took it.
And as someone mentioned, due to the trade, that was $21,000 I did not have to pay sales tax on.