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
![Wink ;)](./images/smilies/icon_wink.png)
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.