How do I check if a remote website has a custom 404 page or not in php?

I am trying to create some basic SEO testing tool, now I don't know how do I check if a remote website has a 404 page or not in php ? Something like this

<?php  
  $info404 = "";
  if(404 custome page exists){
    $info404 = " Good ";
  } 
  else{
    $info404 = " Bad ";
  }
?>

Thank you very much.