A conditional tag for checking if we're in the core WP main query + loop.

in-main-loop.php

<?php
/**
 * Conditional to test if we're in the loop of the main query on a WordPress page.
 * Please note that checking `in_the_loop() && is_main_query()` will NOT work in 
 * this scenario.
 */

add_action( 'loop_start', 'my_loop_start' );
add_action( 'loop_end',   'my_loop_end'   );

function my_loop_start( $query ) {

	$GLOBALS['my_in_main_loop'] = $query->is_main_query();
}

function my_loop_end( $query ) {

	$GLOBALS['my_in_main_loop'] = false;
}

function my_in_main_loop() {

	return (bool) $GLOBALS['my_in_main_loop'];
}