One of the problems with the flying text and battle log is that they don't display dot damage well (or at all). For flying text, all dot damage from all sources is merged into one number, no matter how many people or dots that is. For the battle log, dot damage doesn't show up at all. So, if you want proof that it works in general, you can cast it on an enemy while solo and sheathe your weapon. You won't be attacking or casting anything, but it will still be taking damage you can see, which has to be Thunder (because there's nothing else on the enemy). If you want to know it's working on later content, you'll just have to trust that the debuff marker means something.