A few things overall. I’ll try to be succinct due to my own time limits. It sadly won’t go over every detail and outright specifics.
I have a Co Tank that I work with explicitly on runs. He a warrior, I as Dark Knight. ( Used to anyways, really not happy with Drk at the moment and thus have abandoned it as protest. Until they fix it. Switching over to Gunbreaker until they do. ) But he and I operate on a near instinctive level. Whereas others need to communicate often, he and I just synchronize on runs like jaeger pilots and automatically do what is needed in concert with one another with next to no verbal communication. Sometimes it’s kind of scary. But we both use that as a tool to gauge how well we are doing in our fights. If something falls out of sync, we discuss it and go over the details. We both know each others class, so we observe one another and give constant feedback.
This is in addition to frequent communication, and planning with the team that I run with. We break down each others good points and weaknesses and provide feedback to each other, using various end game content as our active yard stick and the speed with which we tackle it.
An example I can use is Hades Ex, and the frequent checks the fight involves. If we as a group are able to burst him down to 3% hp in 1st phase in relatively short order without any echo or other benefits, we know our checks are on point, as is our rotations. We also communicate damage windows, etc to help. Eventually, you run with the same people enough, it kind of becomes second nature.
Another tool is Stone, Sky, Sea for Heavensward back in the day, or more common now, Circles of Answering and The Lawns. I use these to calculate and adjust my rotations for things, to see if I can shore anything up. It’s not the same as a boss fight directly, but ESO uses what is effectively the same method in their game with their own parsing dummies you can place in that games housing.
I’ll also go in solo on some bosses in ex trials ( used to do this a lot back in the day ) to push my limits and see whether I could solo the boss or not just by myself as Drk. And I would use various bosses as trials by fire to push myself and see what I could eke out.
That’s some, but not all of it. But it’s worked for me overall through the years.