One way to check your PHP-Nuke version is to look it up directly in the PHP-Nuke database.

To do this, go to cPanel > phpMyAdmin. Then from the left bar select the database for your PHP-Nuke installation. Browse the table nuke_config and find the option Version_Num. Its value represents your current PHP-Nuke version.

The other way is to use a simple PHP script which will do the above check for you and print the result.

Create a PHP file, for example vcheck.php, inside your PHP-Nuke folder and paste the following code in it:

<?php
require_once("mainfile.php");
if (!isset($Version_Num))
{
  if ($result = $db->sql_query("SELECT Version_Num FROM ".$prefix."_config"))
  {
    list($Version_Num) = $db->sql_fetchrow($result);
    $db->sql_freeresult($result);
  }
  else
  {
    $Version_Num = "Unknown";
  }
}
echo "Your PHP-Nuke version is $Version_Num";
die();
?>

Then you should open this file in a browser to see the result. The path to the file should be:

yourdomainname.com/nuke/vcheck.php