You don't need use any Hex Editor??? ._.
it all depends how the game is managing the savegame.
If the Wii is adding information to the savegame file, or only the game is writing to the file.
And same for the WiiU, it depends if the VC layer is adding information into the savegame file or not.
For every games you want to know if it's compatible or not, that's just common sense to do :
1. make a save on both console with the same game
2. extract both saves
3. compare the folder's content
The third step is the most important one, the step where you NEED some neurons and interpret what you see.
If both console use the same filename ! Success !
if both console use different filename, but same file count : Maybe they are the same file but with different filename ? success ! just rename it and try
if both console use different filename, but not the same file count : maybe one console use additional files (savestate, multi-save slot, text list, database, etc.), there is necessarily the game's savegame data somewhere into that file ! one of them maybe has the same filename, one of them many has the same filesize ! etc.
all you need is some common senses and some interpretation capacity.
If you have the same filename, and after trying it on the other console it still doesn't work : maybe the format is different ! or maybe there's a console information (header?) in the savegame file.
Open the files in a hex editor and compare them. only here you need an hex editor, at the final step, after everything else failed.