On a spell that gets cast, do I roll to see if I steal a dispel dice right after or after my opponent tries to dispel It says to do this after the spell has been worked out and I figure it doesn't get worked out until after being cast, a dispel attempted and then the spell is resolved if it goes off or not.

codex pg 73 "...after its effects have been resolved..." i take this to mean that after you cast and opponet attempts to dispel and fails THEN you roll the sneaky stealin' die.

Yep, you've got that right!