If you have enough mana to cast the Healing spell, there should be a sound effect and animation (blue effect around character and rotating ankh symbol above), whether it heals or not (and the only time it shouldn't heal is if you are already at max HP), independent of the skill level.
If you don't have enough mana, red text will scroll up the screen by the character saying "I need to recover magical energy" (with a couple second cooldown before it will be shown again, if you repeatedly right click).