Get a notification when users flag your e-mail as spam.
Start here: http://feedbackloop.yahoo.net
Login with a Yahoo account, which can also be via a Google account since it supports OpenID (but to use OpenID, sign-up from the front page of Yahoo.com...)
You then need to fill in a form to subscribe to the service, and it will request to confirm your e-mail address.
Yahoo is very insistant on using DKIM.