Ok, to be clear, when looking at your character stats, the only listing is SPELL DAMAGE:
However, there are pieces of gear that use a different stat called SPELL POWER.
Unfortunately, it's not that easy to quantify since there are procs that only work "while channeling/casting" such as with the 5 piece bonus of the Soulshine set.
I have heard many a rumor, I have never seen anything definitive.
*** NOTE *** I welcome anyone with a good explanation to chime in, however, I would love to see the Dev's to confirm, as I have not since early release seen or heard a reasonable explanation.
Thanks!