Funny I asked the same question and I have a bad habit of not replying to
the list, but Gavin graciously replied to me. I hope you don't mind Gavin,
but
I am posting a copy of your reply to me.

Neil
>How do you know? Is it the AC at the end?

This was Gavins reply:

the first three "octets" are assigned to a specific vendor (actually I
think it's also a valid UPC code).
Do an AltaVista search on "00604C" (or whatever it was) and "ethernet" and
you'll find lists of all the assigned values.  the last three octets are
assigned sequentially by the vendor.
In some cases the information can be misleading if one vendor manufactures
the components for the lan interface in another manufacturer's product.
Generally the MAC addresses are assigned closer to the point of final
manufacturing though, so chances are better that the company who owns
the MAC number will also have their name on the outside of the box (or
on the lan card for most PCs).
Gavin