Detecting when a unit begins casting a spell

how do i do this (when the animation starts playing, etc)?

EVENT_PLAYER_UNIT_SPELL_CAST seems to happen just before SPELL_EFFECT - casting time elapses and the caster plays some of its animation before SPELL_CAST fires. didnt it use to fire when casting time started?

That depends on the spell: some of them works as you expect, but the channeling ones are just a pain in the ass (i.e. Blizzard).

If I remember rightly, you can make it work with EVENT_PLAYER_UNIT_SPELL_EFFECT setting in the channeling spell the "wait cast" field to 0.

Moyack already said it, however, the "wait cast" time is something you already should have in mind.

Might be possible to check for the order as well, depending on the type of spell...