Thanks!
My E38 was actually the rare (and rather underpowered) 730i. Contrary to popular belief, this was actually a small-volume V8, not a V6, with 218 of bhp.
Apart from the rear, where the difference is obvious and striking, the key to telling an A6 and an A8 apart is to look for the "muscles", the power folds in the hood. The A8 has more exaggerated power folds and the headlights are angled differently, making the A8 look more mean or sinister...in other words, cooler. I still mix them up, even though I own an A8! If you go "oh, look, an A8 but it looks kinda whimpy" then you've just seen an A6! ; )