I think it would be great to see for which skills you have enough mana after using one of your spells. This could be triggered in the same way that cast range indicators are triggered. Meaning, while hovering over with the mouse, or while the skill is selected but not casted. For example, you have 230 mana on Pebbles, you select Stalagmites --> Chuck becomes tinted blue indicating that you don't have enough mana for both Chuck and Stalagmites and vice versa.