Check If Custom Post Type

is-custom-post-type

/**
 * Check if a post is a custom post type.
 * @param  mixed $post Post object or ID
 * @return boolean
 */
function is_custom_post_type( $post = NULL )
{
    $all_custom_post_types = get_post_types( array ( '_builtin' => FALSE ) );

    // there are no custom post types
    if ( empty ( $all_custom_post_types ) )
        return FALSE;

    $custom_types      = array_keys( $all_custom_post_types );
    $current_post_type = get_post_type( $post );

    // could not detect current type
    if ( ! $current_post_type )
        return FALSE;

    return in_array( $current_post_type, $custom_types );
}

if ( is_custom_post_type() )
    print 'This is a custom post type!';

Credit:
https://wordpress.stackexchange.com/users/73/fuxia
https://wordpress.stackexchange.com/questions/6731/if-is-custom-post-type/95906#95906