Just yesterday I was trying to figure this out, too! When you get a notification that somebody commented on your video and you click the notification it puts that specific comment near the top with "linked comment" next to it so you know that that's the comment the notification was referring to. You can even see up in the video URL something that looks like this: &google_comment_id=z13xzpdq3njreqbskvzqz3rhbwmexyls