One thing you should look out for is TRD posers. Some people just slap the TRD stickers on there, but they are not TRD equipped. To Identify a true TRD look at the rear diff. It should have a rear locker. That is the give away item, up until today I have never heard of the e-locker on non TRD trucks. A local Dealer should be able to look up the VIN and tell you if it is TRD.
It's a Toyota, and few Tacos have problems. One predominant problem in the Tacoma's is the "Dash Rattle" and should be listened for during the test drive. There is a dash clip that commonly breaks just to the driver's side of the Passenger airbag, that becomes dislodged, and causes the dash to rattle, many people throw plastic tubing between the window and the dash to prevent the noise as opposed to repairing the 98¢ clip.
If the Tacoma has a tow package I would also look for frame damage. The rear of the Taco's frame is a C channel, and an improperly weighted trailer or bed for that matter could bend the frame. An easy way to tell this is look at the area between the bed and the cab. The gap should be even.
A common Mod, the K&N air filter, if equipped... remove the filter and inspect the MAF sensor. Over oiling the filter could cause premature failure of the MAF. Just use common sense.
Hope that helps!