The problem with the A380 specifically is that the machine is soooo big it is hard to see the registration number. The serial number is also on a plaque inside the aircraft, but I haven't a clue where one would go to find it on the A380 so it's easier to look up by registration number or ask an FA. FA's mostly don't know, but the A380 is new enough to be different.

This isn't a geeky question if we're all so interested in it. of course, we might all be geeks.