Magento: Check if any particular customer is currently logged in

require "app/Mage.php";
umask(0);
Mage::app();

$collection = Mage::getModel('log/visitor_online')->prepare()->getCollection();

//Get all the customers that are logged in......
foreach($collection->getData() as $cust) {
        echo 'Customer ID: '.$cust['customer_id'] . '<br/>';
        echo 'Last URL visited: '.$cust['last_url'] . '<br/>';
        echo 'First visit: '.$cust['first_visit_at'] . '<br/>';
        echo 'Last visit: '.$cust['last_visit_at'] . '<br/>';
        echo '======================<br/>';
}

//Get any particular customer, if he's currently logged in or not.....
$collection->addFieldToFilter('customer_id', 5)->addCustomerData(); //5 is customer ID of customer you want to check
if($collection->count()) {
    echo 'Customer is logged in';
} else {
    echo 'Customer is NOT logged in';
}