Areas That Rust:

Look closely for rust in the rear cab corners, bottoms of doors, drivers floor board, the front of the fuel tank where the plastic skid plate is, above the plastic liner in the beds wheelwells (rusts heavily in the bed side above the wheel wells), the rocker panel behind the rear wheel near the rear bumper, the lower corners of the radiator support under the radiator, under the rubber window sills, in the corner of the floor and the rear cab wall, the bottom of the tailgate, and the rear spring shackles. A couple more spots that are prone to rusting are the roof corners, under the gas pedal,  the seam where the cab floor meets the firewall sheet metal, and right smack dab in the middle of the doors, look for rust bulging/bubbling. Check the floor for wetness on either side of cab floors, and down beside the fusebox/kick panels/computer for water dripping, signs of a poorly done windshield, or one that leaked a bit and was sealed up with silicone

Things To Check:

You should take the vehicle to a reputable mechanic before deciding to buy the Ranger to have it checked out.  When initially evaluating the vehicle, be sure to check the following things:

Transfer case for cracks or holes

Differentials for cracks or holes

Bent or dented driveshafts

Any visible under carriage damage

Fluid leaks

Signs of fresh repairs

Wet passenger floor from bad heater core

Ball joints - lift the vehicle and try to rock the top of the tire in and out.

Be cautious if the vehicle is running when you arrive to look at it.  Turn the vehicle off and let it set for a while why you look it over.  Also be concerned if the engine is warm when you show up to look at it.  This could be a sign that there is problems with the engine or hard starting.  When you start the engine, have someone look for smoke from the tail pipe.  This could be a sign of bad rings or seals.  Check the engine fluids.  Check the transmission and make sure the fluid isn't dark and have a burnt smell.  Check the engine oil to see if it looks milky white.  This is a sign of coolant getting in to the engine from a bad head/gasket. If it has manual hubs, check to see if they lock and for any cracks or damage.  When you drive the vehicle, engage the transfer case and test the 4wd. Place the case in 4Lo because the vehicle will creep and you'll be able to feel the low gearing.  4Lo is an easier range to FEEL if the vehicle is in 4wd than 2Hi.  When you drive the vehicle check for strange noises, handling, steering, and check the transmission in all the gears.  If the vehicle has air conditioning, check to see that it works.  Check the settings on the heater and ensure the fan turns at all speed settings.