vin check
i assume you're looking for accident history and the like. you can run carfax but remember carfax is only as good as people who enter the info. Look for gaps in mileage and registration periods. I'd be nervous if ther is a large ,ileage period where the truck cannot be accounted for. You may also be able to get a friendly insurance agent or damage appraiser to run the vin# through the insurance database which will tell you if the truck has ever been repaired on an insurance claim.
look for after factory welds, paint overspray on glass or weatherstipping. also check the body to frame bolts to see if a wrench has been on them.
you can check tracking by driving though water and then hold the truck in a straigh line for a couple hundred feet. when you look at you tiretracks on the pavement the front and rear tracks should very closely overlap.
pull the carpet and check for silt or mildew and rust - flood damage -
good luck
craig