There several ways in which a page can be published. In addition to the user simply activating the page from say the side kick or the Sites console a page can be published through a workflow (either by the author starting a workflow directly, or via the Activate Later function. In the case of activation via a workflow there is a process step at the end of the workflow where code triggers the publishing of the page in question. Be default this workflow code will use that a system user (the admin user) to publish the code.
If you are use the Activate Page/Asset custom workflow step (in the WCM Workflow category) you have the option to choose Replicate as Participant which will tell the workflow process code to use the approving user to publish the page instead of the admin user. If you aren't using Replicate as Participant the only way to know who published the page is via workflow history.
