Don't try to save a couple bucks by looking the truck over yourself. That's why they have mechanics.
First thing to do is contact a garage (a real one, not a gas station with a bay)
Make an appointment to have the truck inspected, with the instructions you want to know EVERYTHING that is wrong with the vehicle, and what it will cost to get it fixed.
Either take the seller with you, or take the bill back to the seller and you will probably be able to get the seller to knock the amount of the repairs off what he was asking. Whether he will pay for the needed repairs or not, at least then you will know what you are getting into. If you decide then you want to buy it, before the deal is finalized, take it to a transmission shop and have the fluid changed and a new filter installed. This will probably cost you $60- $75. It needs to be done anyways, and if there is any problem they will find it. Get the sellers permission before doing this, and if they say you're going to need a new transmission, walk away from the deal before you get into it. If you do this it will cost you $100-$150 which you can probably get the seller to pay, if the mechanic can't find anything., you'll know you're getting a good deal.
http://trucks.about.com/od/usedvehiclebuyingadvice/Used_Trucks_and_SUVs_Buying_Advice.htm
http://www.hsssearch.co.uk/stories/articles/-/lift_trucks/lifttrucksafety/buying+a+used+truck%3F+insist+on+thorough+examination/