"Original" AMG claims are easy to make, and done to gin up prices as a trap for unwary buyers.
Easier to do since all "Pre Merger" original work orders for the Affalterbach built AMG cars have been discarded by the time AMG was bought out by Mercedes Benz and merged into MB in about 1998;
The chances of finding a DOCUMENTED AMG car are low.
This is wholly apart from the AMG put together cars from the authorized US AMG factory facilities in Florida, Chicago (the Westmont Illinois AMG mother store) and Bevery Hills Motoring Accessories.
And "put together" versions like mine, which was built from individual AMG parts, such as the headers, downpipes, wheels etc.
This subject has been discussed with regard to the Generation I (1980-1985) and Generation II (1986-1991) W126 S class AMG cars to death on the Benzworld W126 section ..
In fact, people on there are constantly combing cl and autotrader and elsewhere, for AMG cars for sale and ever one that shows up, is the subject of a heated discussion of ts AMG provenance, and dubious and spurious claims are usually outed immediately.
Here is an example of a current typical discussion, on such a car,
gen2 euro? with AMG bits on theBay - Benzworld.org - Mercedes-Benz Discussion Forum (I personally know some of the posters)
Mercedes-Benz : 500-Series Mercedes-Benz : 500-Series | eBay
On this one, for example, and the one rreferred to by the OP, I think the chin spoiler is NOT an AMG original, and was probably replaced with whatever else was available, ie, Wald, Lorinser, etc, because the orignal chin spoiler, if it was an AMG one, was likely replaced due to the fact that on the lowered cars they frequently were smashed on parking stops and curbs and moderatey steep driveways by drivers that did not slowly exit on the oblique which is what you need to do..
And sometimes were replaced with --- whatever.
As to how do you know it is a true 6.0 liter AMG motor, the first thing I would do is take off the valve covers and look for AMG stampings on the cams.
As an example of this kind of inquiry, see the opened up AMG 32 valve motor pictures here:
Picasa Web Albums - Blue Ridge Mercedes - AMG 5L 32V