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 (php,JavaScript,jQuery,HTML) if their are others doing this?
Is this allowed on the internet for them to do this and can I take action?
HTTP access can be blocked to some extend by using an HTTP Referer filter. Access "by host server" can also be monitored through looking at the Referer in the HTTP logs. It's not a perfect solution, but for standard browser access will get you most of the way there. ("No Hot-Linking" setups sometimes work like this.)
For legal action, seek the advice of a lawyer :-) However, my first inclination would be to ask the other site owners to stop. They may just be nice.