A portion of the 3.0 V6's had head gasket issues. You can take the VIN to a Toyota dealership to see if its HG was replaced in the recall campaign.
As far as a general inspection, rust can vary dramatically. Make sure there is no excessive rust underneath, especially in those important places such as the frame. And I know this sounds obvious, but drive the truck on the highway at high speeds. This will show any high speed vibration, noises, rumbling, etc.
As long as your HG is good, the V6 can last a long time. Not the best gas mileage, but it will last. If the truck has a solid frame and a strong motor, start talking numbers!! Post some pics too.