Var dump the value of a post meta data

var_dump_post_meta.php

<?php
add_filter('the_content', function( $content ) {
  //Replace 1 with the page ID you want to test in and the custom_field_name with your own
  $page_id = '1';
  $key = 'custom_field_name';
  if( is_page( $page_id ) ) {
    var_dump( get_post_meta( $page_id, $key) );
  }
     
   return $content;
});