How do I spot heavy towing use on a used diesel truck?

Look for evidence instead of relying only on the seller description. Heavy towing clues include fifth-wheel or gooseneck hitch hardware, bed rail holes, worn receiver parts, trailer wiring repairs, brake controller wear, sagging rear suspension, uneven tire wear, hot-spotted brakes, transmission service gaps, coolant stains, and extra gauges or tuning. Some towing is not a problem if the truck was maintained and set up correctly. The risk is heavy use without records. Ask what was towed, how often, what the loaded weight was, whether transmission and differential service were done, and whether cooling or brake repairs were needed.