Look for a truck with a factory hitch receiver and a 7 pin connector NOT mounted to the hitch. Look for a factory Trailer Brake Controller. That will tell you it was bought set up for towing. Rear axle ratio can be determined by the axle code on the white sticker on the driver door frame. Also it’s GVWR. Look at the yellow sticker for Load Capacity. If it’s a GM from the last few years, they actually have a tow capacity sticker!
I recommend finding a truck or two you think might work, take photos of those door stickers, and use them for some online research. The meaning of each of the codes on the white federal weight sticker can be found in the online owners manuals. There are some posts on the F150 forum on how to get the factory window sticker for a given Ford truck.
Keep in mind the more options or upper level trim series, the more you lose truck payload and with that tongue weight carrying. So be mindful of those yellow stickers.
Let us know what you find, plenty of free opinions here ;)