When you get manage_pages permission from a user, you can access the list of pages on which they are located admin /<user id>/accounts - this is the only way to check whether the user is the administrator of a particular page if your application does not access them as tab on this page.
In this case, the signed_request sent with the downloads of the tab contains a page element telling you if the current user likes / is the page administrator, as well as the page identifier
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