How to check if GD Library is installed?

Try this:

php Code: 
<?php 
if (extension_loaded('gd') && function_exists('gd_info')) {
     echo "It looks like GD is installed";  
}
?>

You could also spit some info about your GD version by doing this:

php Code: 
<?php
  echo "<pre>"; var_dump(gd_info()); echo "</pre>";
 ?>

 

You will definitely get the ans of your GD Library is installed or not.