The page_exists tag checks whether or not a cloned page, the name of which is given to it as a parameter, exists.
It returns a '1' if the page exists else it returns a '0'.
<cms:page_exists 'contact-us' masterpage='index.php' />
The above snippet is checking for the existence of a page named 'contact-us' that has been cloned out of a template named 'index.php'.
Name of the cloned page the existence of which is to be checked.
Name of the template out of which the cloned page mentioned above has been cloned.
If this parameter is skipped, the name of the template of the currently executing page will be used instead.
This tag is self-closing and does not set any variables of its own.