I would try to find one that has been used primarily as a personal vehicle, preferably one with towing and plowing prep packages. You can usually tell by looking at the bed how hard it's been worked, but bed liners can mask that, and some trucks may have towed a lot (i.e. camp trailer haulers) and show little use of the bed. Some used car dealers will actually put a new plastic bed liner in their used trucks to make them look like they haven't been worked.
For example: I bought my work truck new in 2003 and it only has 52K miles on it, but, almost all of those miles are towing, hauling and plowing locally, which causes a LOT more wear and tear than highway miles. I've got almost 4,000 hours on the hour meter, putting my average speed at around 13 mph. From the perspective of someone buying it used, that's not good. It would be better to find one that has 100k miles highway miles on it.