I think there should be some kind of an indicator for polls on forum display. Maybe a little icon attached to the right bottom corner of the thread opener's avatar. Something like this, but smaller:
Yeah, I once used prefixes for sticky and closed threads. Now you mentioned it I might just do it for polls and have it just display in the forum_view area since thread_view imo won't need the prefix since it'll be obvious a poll is created in this area.