I have a fb app. I want to get the webbook response to my app, whenever a user tags a particular page (of another user). how to do this?

I have a fb app. I want to get the webbook response to my app, whenever a particular page(of some other user) is tagged. How to do this? What permission do i need to take from the page? Is there some other way to do that also get any kind of response to the app, whenever a particular page is tagged?