Most tank busters are 'cast' by the boss and you see the cast bar over their HP bar. As you get into higher tier content (mostly only in savage primals/raids) bosses will sometimes just do heavy hitting moves with no cast bar and you just have to learn when they happen. Sometimes the names are pretty straightforward: "Halicarnasus is casting Critical Hit". That sounds like a single target ouchy incoming, and it is. Some are less obvious. "Exdeath is casting thunder 3" well that sounds like it could be a raid AOE, or a single target, who knows. Its a buster.

Online guides will spell it all out for you for any specific fight. But if you go in blind, then a safe bet is if you aren't sure what they are casting, pop something. Better safe than sorry and if your HP drops a WHOLE lot then you know for next run, if it hardly budges, or the raid as a whole takes a buncha damage, no biggie. You know that for next time too. Until you hit savage content, you wont really run out of cooldowns doing this so feel free to pop one wheneer you aren't sure and take note which are busters and which aren't. You can always fall back on your hallowed ground if you run out of CDs while you burn them learning.