Joking aside, if you see an attack with a cast bar then it's either a raidwide, a TB, or a mechanic. There's sometimes no way to tell which is what until you eat it, but in dungeons it's rarely lethal anyways.

Cues can be found in the animations though!
An enemy's arm glowing red can indicate a one-sided aoe, a long windup can indicate a TB, etc

As for harder content, guides often give you the attack names and their effects, so you just need to memorize them.