I think there are two answers here - around 2006ish manufacturers made it much more difficult (impossible?) to hide if a truck had been modified. So, with a newer truck like the one in the thread I would think he could have taken it to the dealer before he bough it and they could have told him. I'd hope that since the dealer will deny warranty coverage they'd at least have the decency to tell you beforehand.
As for the older trucks they're out of warranty so I suppose it's a moot point. As for knowing just for peace of mind I don't think there's a sure way. Of course, looking for gauges or holes where they were is a big clue.
