I'm not sure about Mercedes, but I can spot Mexican or Brazilian-built VW parts a mile away.
If MB parts are subject to the same standards as the VW parts, here's where the discrepancy occurs:
1. Quality. You want to add fuel to the "quality" thread, take a look at fit and finish. If the parts are German, then there should be no degradation of the true MB quality.
2. Markings. German "knock-offs" made anywhere but in Germany will be stamped as such..."Hecho en Mexico", for example. I believe MB requires this.
3. Longevity. 15-year old MBs hold their age well. 15-year old knock-offs will not.
4. VIN. MBUSA should be able to tell you the rest of the vehicles assembly history based on that...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car