Beyond what Rongway said about looking at the shape of the arena and any repeating patterns(be they on the floor or on the outside since sometimes attacks come at you from out of bounds), i also pay attention to the boss(and the other players if im unsure). For me this means first setting a Focus Target on the boss. My UI has the focus target info in the part of the screen i focus on to keep the timing right for my rotation, procs, and cooldowns. Sometimes the name of an attack will indicate where they attack will land, and even if the name is basically gibberish i can try to remember that "Aetheroplasticene" is the attack that does _____ after i see it a time or two. And then beyond that, i try to watch the boss for physical tells. He appears to be winding up for a big attack with his tail? Then id better get in front/to the side.

And when all that fails, if i dont get it after a run or two then ill watch a guide.