I don’t know about Ford and Chevy, but Ram puts out a towing capacity chart based on each truck’s exact specs. Here’s an example:
So if you can find out all the info about the truck needed for the chart, you can find the exact towing capacity per the manufacturer.
Also, most modern trucks (like for the past ~10 years or so) will be wired for towing with an in dash brake controller and a hitch installed by the manufacturer. If it’s a newer truck without that, I’d be automatically skeptical. (Of course this doesn’t apply to much older models)