If you use Gmail you can find out which websites are sharing your email address without telling you. Imagine for a moment that your name is April Anonymous. When entering your email address, after carefully considering whether you should at all, add ‘+websitename’ before the @ symbol in the address. All email sent to April.Anonymousfirstname.lastname@example.org will still come to April Anonymous’ inbox, but if any unsolicited messages show up you’ll know precisely which site shared your email address.
If you’re interested in checking up on spam offenders in a more detailed way, use a throwaway email service that lets you create multiple inboxes (Inboxen works well for this) and set up a different address for each service you sign up to. Then watch the inboxes carefully to see where messages from services you haven’t signed up for start appearing. There’s your culprit.