You can use the following code to check Elementor Builder has used for a page.

if ( Elementor\Plugin::instance()->db->is_built_with_elementor( get_the_ID()) ) {
    //page is build with elementor
}

// or by checking directly
$elementor_page = get_post_meta( get_the_ID(), '_elementor_edit_mode', true );
if ( ! ! $elementor_page ) {
    //page is build with elementor
}