Check if the current post is a particular post type.

check-custom-post-type.php

<?php

/**
* Check the current post to see what type of post it is.
*
* This will return true whenever a post is equal to a specified post type.
* It can be useful when making global changes for a particular post type.
*
* @param   $post_types array or string the type of posts you're wanting to check.
* @return  bool true if the current post type matches any of the checked types.
* @author  Robert Neu <http://wpbacon.com>
* @author  Gary Jones <http://gamajo.com/>
*/
function prefix_is_cpt( $post_types ) {
	if ( in_array( get_post_type(), (array) $post_types ) ) {
		return true;
	}
	return false;
}

example-usage.php

<?php

/**
* Do conditional stuff based on what post type the current post is.
*
* @uses    prefix_is_cpt()
* @author  Robert Neu <http://wpbacon.com>
*/
function prefix_do_stuff_for_post_types() {
	if ( prefix_is_cpt( 'page' ) ) {
		// Do something for pages.
	}
	$post_types = array(
		'post',
		'review',
		'tutorial',
	);
	if ( prefix_is_cpt( $post_types ) ) {
		// Do something else for these types.
	}
}