I think it would be very helpful if there was a way to get statistics about your own API keys.
That way, you could find out which IP addresses were using your app the most. (And if one was abusing it, you would at least know where the problem is.)
The way it stands now, there is no way to find out who is using your app's key.
Update: Before anyone gets carried away here, I want to point out that this is not a dupe. This question is not asking whether I can check the number of requests made using my key. I know how to do that. I'm asking how I can determine the number of users using my key - and how much they use it.