Interestingly enough, 3 responses and 3 different approaches. It seems to comes down to personal preferences.

Add on's: not interested but might the best option obviously
Casting bar: I don't prefer to track my eye all the way to the top of the screen but will experiment with shift F.
Visual Cues: It's unreliable for me because I use Legacy Camera, the display is not always focused on the boss all the time.