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The bad thing about relying on badges, mirrors, etc. is all that stuff can be changed easily. A lot of people try to determine the early 99's by the badges being on the fenders and not the doors... well my early 99 has OEM door badges (yes, some of the early turds had door badges).
Another easy way to tell is to take a can of gasoline with you- start the truck and set the can of gasoline up near the front. I've heard that a 6.0 will stop running instantly if it even smells gas
One way (I think) to tell the early birds from the others is the style of the airbox. It's difficult to explain the difference, but the difference is in the way it opens.
At least the late 99 that I bought and the early 99 I looked at had different airboxes.
If it is stock you can tell by the badges but when you look at the engine you can tell imediately. There is several trucks now that look exactly like an 05 since all the parts are interchangable.
Easiest way is to look at the engine (mirros and badges can be changed like superduty4x4 said). If there is a black dressing cover on top of the engine that says Powerstroke on it, then you're looking at a 7.3. If it says Powerstroke V8 Turbodiesel on the fan shroud then you're looking at a 6.0 (just like superduty4x4 posted).
If you go by mirrors, turn signals started with the 6.0 (NO 7.3s came with turn signal mirrors from the factory).
If you want to go by factory diesel badges, here's what each year's door/quarter panel badge looks like: