How can you tell when your images are being used without your consent?
There are many faulty systems that can check for image similarity, you've already got a variety of answers regarding their mixed results. Those systems don't take fair use into account anyway.
If you're hosting images on the web, you could programmatically check for hotlinking, but the damage is done. When you put something on the internet, you've lost all hope of ever keeping people from using your stuff,
and that's the beauty of it all.
That's what the internet is all about, sharing stuff you made and letting other people share it. To think that you can stop people is incredibly vain. Not everyone lives in your country, and therefor not everyone has to comply with the copyright laws of your country.
If you do find someone using your work, ask them nicely if they'll at least cite their sources. It's the polite thing to do, and polite people will often comply.