Whoa... I think I answered this question a week or two ago.
Something like Antagonist or naturEnemyCastingBar will show you when buffs are cast an how long they last. But unless you know what the buff the icon stands for... Witch hunt MIGHT tell you when buffs are cast.... I don't recall - I don't use it, as all those words at the top of my screen are annoying.
But to actually see their buffs in the buff slots on the target unit frames.... You need a mage to cast amplify magic.