plugin recommendation - Hook to See if Comment got a Reply? in WordPress

I figured it out. Each comment row in the wp_comments table has a field named comment_parent which stores the comment_ID of the parent comment. If the comment has parent, then the default is 0.

So I guess on each comment post I would check the database for the posted comment's parent, and if it matched the comment of a subscriber,tify him or her.

Sounds like a plan.