Depends on what you intend on using it for. 2WD vs 4WD? 4 cyl or 3.0/ 4.0 V6's? Regular cab, extra cab or 4 door? Older ranger or newer one?
Aside from that watch for rust under the doors and along the bottom, make sure the head gaskets aren't blown, common sense stuff that you would look for on any used vehicle. You may also want to run a carfax on anything you're interested in to make sure it isn't salvage titled or anything like that.
We'll need more info in order to help further.