Getting the commenter cookie data

Whats the best practice for getting a commenter’s email address from the cookie? Part of my plugin relies on having an email address (sort of like gravatar) and I’d like to try to get it even if they are not logged in by checking for what they used in a comment cookie.

To be clear: I may need to get this at any time, not just when making a comment so I need to get it from the cookie rather than a comment object.