This code expand the capability of is_lifterlms() and is able to check if the sale page that is loaded is included in some course or membership.

llms.template.functions.php

<?php

// Espandend to use is_sales_page()
if ( ! function_exists( 'is_lifterlms' ) ) {
	function is_lifterlms() {
		return apply_filters( 'is_lifterlms', ( is_courses() || is_course_taxonomy() || is_course() || is_lesson() || is_membership() || is_memberships() || is_quiz() || is_sales_page() ) );
	}
}

/**
 * Is Sales Page
 * @return bool
 * @since 3.0.0
 */
if ( ! function_exists( 'is_sales_page' ) ) {

	function is_sales_page() {
		$post_id = get_the_ID();
		$args = array(
			'post_type' => array( 'course', 'llms_membership' ),
			'meta_query' => array(
			   array(
			       'key' => '_llms_sales_page_content_page_id',
			       'value' => $post_id,
			       'compare' => '=',
			   )
			)
		);
		
		$query_meta = new WP_Query( $args );

		if( $query_meta->post_count > 0 ){
			return true;
		}

		return false;
	}
}