You can check whether a PHP function is enabled by using the function_exists function in a simple PHP script. For example, if you wish to check whether the fsockopen function is enabled, you can put the following code in a php file:

<?php
if(function_exists('fsockopen')) {
echo "fsockopen function is enabled";
}
else {
echo "fsockopen is not enabled";
}
?>

Then you should open the file in a browser and see the result :)

Please note that on SiteGround's servers there are no blocked/disabled PHP functions.