I think the question is: How can you tell without self reporting to Getty.

One way that's likely to help you discover many (if not all) getty images is to install Picscout (Getty's) image exchange.
http://picscout.com/imageexchange/home If you download this free add on, you can browse every page of your site it will load your images and compares images on your site to the ones in picscout customers catalogs. When operating properly, the add on opens a sidebar and loads the image in the sidbar. It's a little slow-- but you can step through your site this way.
I don't know if every or even most of the images in Getty's catalog are detectable using this agent, but you can at least find the ones that can be detected by the Picscout agent. Then you can take them down.
The obvious flaw with this method is that if you do it, we don't know whether Getty will then send a human out to look at your site. The other obvious flaw is that you don't know what happens if a getty image is detected but it's not participating in getty's 'imageshare' program. Maybe Getty records that but doesn't show it in your sidebar.
(BTW: some of the false positives are hilarious!)
I would advise that if you use this too, you do it at a time when you can very quickly remove any true positives from your site and remove them
right away. By right away, I mean: if you find on on "page X", remove it
before checking "page X+1". Also, block access to the wayback before you do this.
This is a bit paranoid, but we don't know what Picscout/Getty is going to do with information.
After you check your site, and remove any suspicious images you find, I advise de-activating the add on. No point in helping Picscout/Getty find more violations at other people's site. Well... I guess unless there is no point unless someone is a super-mega enemy you hope to have Getty pester; but even then surfing through their site isn't guaranteed to get Getty after them. And it's not very nice. (Also, having the tool active makes surfing slower. So, it's silly to keep it on when you aren't actively using it.)
By the way, I'd be happy to provide people with images that result in hilarious false positives.