Are you going to see it before you buy or is it off the internet? As AJ said, all are n9 body clones, none of the meego software.

On the dialler, type *#0000# and it should say:
Software version Vxxxxx_PR_00X
N9
(C) Nokia

Also check the IMEI begins with a 3, as this usually denotes Nokia.

Otherwise, make sure it functions how you expect the N9 to do so i.e. swiping, no lag, same vkb etc.