It depends on which skill you are using for the spell. If you are using a skill that grants a temporary buff like Avatar or Berserk, you can either detect the effect of the ability or the buff on the casting unit. If you are using an attack enhanching skill like an arrow, you can either detect the order to enable or disable auto-cast or use a damage detection system with the arrow's target buff.
First tell us which skill you are using for this purpose.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory