Quick snippet/plugin/dropin to test IPN support

ipn-test.php

<?php
/**
 * Plugin Name: PayPal Sandbox IPN Tester
 * Description: Pings the IPN endpoint to see if your server can connect. Just head to <a href="/?ipn-test=1">yoursite.com/?ipn-test=1</a> whilst logged in as admin.
 * Version: 1.0.0
 * Author: WooThemes
 * Requires at least: 4.1
 * Tested up to: 4.3
 */
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

add_action( 'wp_loaded', 'paypal_sandbox_ipn_tester' );

function paypal_sandbox_ipn_tester() {
	if ( ! empty( $_GET['ipn-test'] ) && current_user_can( 'manage_options' ) ) {
	  $response = wp_safe_remote_post( 'https://www.sandbox.paypal.com/cgi-bin/webscr', array(
	  	'body'       => array(
	  		'test_ipn' => 1,
	  		'cmd'      => '_notify-validate'
	  	)
	  ) );

	  if ( ! is_wp_error( $response ) ) {
	  	wp_die( 'SUCCESS' );
	  } else {
	  	wp_die( 'FAIL - ' . $response->get_error_message() );
	  }
	}
}