You can always watch streams and VODs of people that play your class and try to mimic what they do. This can be helpful to try to understand their decisionmaking, and eventually you'll start to pick up on the rules of thumb they're using.
But the real answer, as unsexy as it is, is trial and error. I'm still learning how to use my cooldowns, and I'll be learning how to use my cooldowns for as long as I play WoW. The introduction of M+ has really hammered this point home for me because the things that worked in a +5 won't always work in a +10, and the things that worked in a +10 won't always work in a +15.
WoW is often a game of “good enough”. You learn how to heal good enough to survive a certain boss. You learn how to DPS good enough to kill trash before you run out of crowd control. You tank good enough to stay alive for four minutes on Hyrja
. But as you keep cranking up the level of your keys, “good enough” stops being good enough. And then you have to figure out how to get better.
I've probably healed Melandrus in Corte delle Stelle
with six different pairs of legendaries as I've gone from M0s to +27. Eventually Melandrus does more damage than I can heal, and I go back to the drawing board. I try different trinkets, I try different combinations of healing cooldowns and externals, and I pay attention to which DPS classes need more and less help from me.
For now, I'm pretty confident that I can heal Melandrus “good enough”. But maybe we get that juicy +29 Corte delle Stelle
key and everyone dies on the fourth Maelstrom Tagliente
. Then I get to start learning when to use my cooldowns.