Edit: You have to create a new account to see at what point you have been hellbanned. E.g. https://news.ycombinator.com/threads?id=phillijw with showdead you can see that all his recent comments are dead, without login it will only show the comments he has made when still alive.

Also you can't get downvoted when you are dead so if you stop receiving downvotes, you either have been writing excellent comments or you've been murdered by HN assasins.