from the wad file itself you can do only one thing:
unpack the wad and check the 0000001 -0000006 files. if the manual (html) is the same and the rom is the same (compare with a downloaded one) then probably it's original.
it's hard to do anyway, you need tools and time.
otherwise you could install it on you wii and check if banner-game-manual refer to the game. banner usually isn't a logo, it's just the start screen of the game.
if the game is the same in all 3 it is (probably) the original wad
so i take a look at the emanual.arc file, if the file is modded or not.
is there somewhere a list with the ticket codes for the wad files and games? because, i think its bad to give a tiket name whichs already used by a other wad...it overrides my savegames, or was i wrong?
Install the Wad, start it and see if there is an manual and if it is corresponding game.
If you want be 100% sure, just make an CRC Check. But the method with the manual is fine enough.
sjeps suggestion is too complicated.