Vasir

Id say the easiest visual is the front grille and badges. other than that, my S320 and S600 look identical if the badges on the s600 was taken off and grille replaced with a s320/s420/s500 grille (my s320 is debadged).

also if you look inside the s600, the seats will be two toned. and the shifter will have 'V12' on it. THe dashboard will have leather stitchings and suede headliner, but that will be difficult to tell the diff unless u knew exactly what the normal headliner looks like...so i say the easiest way is the two-tone seats, dashboard, and v12 marked shifter.

6_0_0

I still believe what you are recently saying is a bit wrong... In the very first years of production there were no external differences at all (except for badging). Even the grille was the same. And do not forget that S600 are both SWB and LWB. The V12 on the gear comes later, and the seats do not have to be two tone, AFAIK. And one more thing - a lot of info here is based on American experience, while eg. European cars can be a bit different.