check cab corners, wheel arches, other known spots for rust.
listen for any weird noises when starting (start it cold if you can), any weird noises while running.
pull codes if you can
carfax
ask for a maintenance log if they have one
check frame and underbody for rust and what not
get brands on all the add ons (sound system, wheels, tires, lift, any after market parts)
if it's a drop in bed liner, check underneath it.
lots of stuff u can do for free...
oh and it's worth driving to see it... but i'd get some more advice on here first as well. obviously you want to see it before you commit to buying it.