look for tool marks on stuff that typically gets replaced with performance goodies, EG the EGR system, exhaust flanges around the down pipe, cat, etc. look for screw holes or 2-sided tape/velcro where chips/tuners might've been plugged in under the dash, excessive wear to the OBD-II port (tuner being plugged in repeatedly), loose OBD-II port (tuner plugged in full-time), and generally just stuff that doesn't look right. holes where there shouldn't be holes, tool marks on the radiator support, stuff that shouldn't need to be removed, etc. if i open a door and the seat brackets are rusted, i'm walking; it's only a matter of time til that cancer gets in the floorpans if it's not already there. if the coolant is the wrong color, that's a big indicator of the previous owner having a "lawnmower" maintenance program. ditto for trans fluid smelling like cotton candy or brake fluid looking like hot chocolate. loosen the oil fill cap and start the truck; if it starts dancing like a drunk fat girl, that's probably not good.