This could be done by overlaying 1-4 stars on the quest button, indicating the difficulty:
- 1 star: Easy
- 2 stars: Normal
- 3 stars: Hard
- 4 stars: Brutal
For old savestates where the highest beaten difficulty is not known (yet), just display 1 star (just for backwards-compability; the player can always replay the quest to get a better rating).
Bonus points for also showing those stars on the buttons for the 2 quest sets (e.g. Earth), to be displayed when all quests have been won. It shows the lowest best difficulty in the quest set.
So if you have won all quests on easy, you get 1 star, if you have most quests won on hard but one on normal, you get 2 stars, and if you have won everything on brutal you get 4 stars, of course.