How do I tell if my cog needs replacing?
The sprockets (apart from the 13 tooth) are reversible, so you can get twice the life out of one by taking it off and using the other side of the teeth. From the photo it looks to me as if it's only worn in one direction so far, or at least significantly more worn in one direction.
http://www.rohloff.de/en/products/speedhub/sprocket/index.html

(From my experience with a 3 speed hub, a hub gear sprocket can be badly hooked without causing problems when run with an equally worn chain, long after the point where a derailleur wouldn't shift properly. I'm not recommending that you let it get that worn though.)