Once you learn fights you'll also know what kind of AoE's to expect for instance if the fight delivers many frontal AoE's you should crowd his attack box so you're as close to him as possible that way you can run through him faster and have more time to get out of the way.

on the flip side, if it's a circular AoE athat hits everything within a certain radius of him you stay on the very outside of his hit box.

it'll all come with practice and learning the fights. Just know that you need to move immediately, dont finish yoru combo, don't finish the spell or whatever. You see the lines you GTFO asap. Nothing worse for healers than tanks eating dodgeable AoE's.