I think someone is stealing my content using an iframe. My website is a forum and a user has just reported them to me.
How can I find their website programmatically
Use the same method that I suggested here: How to limit display of iframe from an external site to specific domains only
In a nut shell, you add a PHP script in every page (in your case it will probably be just one, assuming it is a template), this script limits the viewing to one (or more) reffering domains.
This method is better than a javascript method because the users might have it disabled.