WordPress at a glance
function is not described

WP_Query::is_preview() WP 3.1.0

Is the query for a post or page preview?

This is a method of the class: WP_Query

No Hooks.

Return

true/false.

Usage

$WP_Query = new WP_Query();
$var = $WP_Query->is_preview();

Changelog

Since 3.1.0 Introduced.

Code of WP Query::is preview: wp-includes/class-wp-query.php WP 5.2.2

<?php
public function is_preview() {
	return (bool) $this->is_preview;
}