If you want to find out whether the given web page is generated by SharePoint or not then the most authentic way of verification is to check Response Headers. SharePoint adds an specific response header (along with others) called
The presence of this header is a proof that the given web page is served by SharePoint. The value of MicrosoftSharePointTeamServices header will be similar to 14.0.0.xxxx where major version 14 means it is SharePoint 2010.
The quickest way to check the response header is to use fiddler . For example,
As any web server is allowed to add any Response Header therefore, please make extra checks if there is a mission critical dependency on this check.