Pages for Cron use Only?

You could use php_sapi_name().

See this demo lifted from the developer docs

<?php
$sapi_type = php_sapi_name();
if (substr($sapi_type, 0, 3) == 'cgi') {
    echo "Your are probably a human";
} else {
    echo "You are probably not a human";
}
?>

You could just apply this kind of conditional at the start of your script.