Personally, when I got my truck, there were a few things that were important when looking at it.
Look underneath the truck..you want one that isn't completely rusted out. Check out the frame, and see if it looks straight..See if there are any obvious leaks that could cause a problem in the long run..
Would want to check out the engine..listen to it run. Don't want to hear and "ping" or "knock" noises.
A big part for me was over all cleanliness of the vehicle. Does it look like the previous owner took goor care of the truck? You can tell...
It's always a good idea to have a professional check out the truck cuz lets be honest, they know more than you or I ever would. Carfax is also a good idea to get on the car!
Good luck with your search! Hope this stuff helps..These are just a few basic things to look at..I'm not an expert by any means but like to think i do have some knowledge