I've never bought a car from a dealer. Figure out what you want, study the hell out of it to figure out what you're looking for (options, trouble spots, maintenance history, etc.). Then TAKE YOUR TIME and find the best one out there. Don't buy one prior to the 6th or so you look at IN PERSON). Nothing can ruin the used car buying process more thoroughly than buying the first one you find.
Don't waste money on CarFox reports, inspect the vehicle yourself.
In newer trucks, check the VIN stickers on all of the body panels to be sure of its history in terms of accidents. Every major panel (body, quarters, doors, hood, sometimes even bumpers, etc.) has a VIN sticker. They should all be present and they should all match. If one is missing or mismatched, it's not a deal killer, just check that panel and those around it carefully for signs of work, filler, panel alignment, broken attachment points, missing/chipped paint/wrinkles etc.
The "weather" should be bright and sunny with some clouds. Road grime is fine, leaks are not. There should be no drips. If it's spotless, it's been cleaned and you have to find out why. A dirty car is an honest car. Most important, you'd better be UNDER that car at some point in your inspection.
Know what came with the car from the factory (this goes back to the studying). Tool kits, cargo covers, literature, etc. In the best case scenario, these are bargaining chips, worst case and you're going ta have to track them down later.
If the truck in question is known to have any recalls or trouble spots, be sure they're addressed and documented. If they haven't been addressed (i.e. timing belts), bake it into your budget. In fact, assume that any factory suggested maintenance that doesn't have a receipt hasn't been done, know the price of these items, and do them within the second month after purchase (the first month is to see if there are any additional trouble spots that need to be addressed at the same time).
Flat white is the easiest/cheapest color in terms of body work. Anything darker is worse. Flat black is pretty bad. Metallics are terrible. Black metallic... You'd better really like that paint.
Finally (and most obvious) test drive it. Check every single feature and function. Know what it's supposed to sound like and how it is supposed to behave (which is why you check out a half dozen before you start looking at the ones you may actually buy). Be knowledgable (or have someone so skilled with you) as to what sounds, rattles, bumps, etc. mean from a mechanical point of view.
Finally, take detailed notes on every specimen you inspect. Compare conditions and prices (be sure to look at a few above your price range for data points and negotiation fodder). Share it with the seller if you think it'll help.