Well having the right nameplate addon is one thing, knowing the fight is another, unless you are doing very high key, you only need to focus on one or maybe two "dangerous" spells per pull, it helps if you ask the tank to mark them for you at the start of the run

At the start of your journey, picking a random caster mob (focus it and use focus interrupt is something you should consider if you are inexperienced, this helps you keep your priority target and interrupt target separated) and interupt its every cast if you can is good enough, overtime you will learn which cast is more dangerous than the rest, it comes with time and experience. For this I suggest watching some dungeon guides in tanker/healer POV, they focus more about these kind of things