How should I indicate to users that Escape can close a modal?

The modal windows on our site have a Close button (X) in the upper right corner, and for years that's been the only way to close them. I just added the ability to close modals by pressing the Esc key. I want to let our longterm users (and our new users, too) know that this functionality is available. Is there a good / standard way to show that hint? Maybe as a title on the Close button? Are there any examples of where this is done well?