The only way I would be able to tell, besides black smoke when in the throttle would be by the tail pipe and the sound. These 6.5s have the sound of a dying dog according to my daughter, and of course the turbo whistle when she’s spoiling up. Otherwise I don’t thing there is any other way unless it’s badged. But he swears up and down he sees them all the time. I asked him to take pictures6.5 powered Tahoes are fairly rare 2 or 4 door.
Ask him how he tells the difference between a 6.5 diesel and a gas powered Tahoe.