How to check if customer is already subscribed to newsletter

Alternatively you can try this, if you have the customer's email address:

$subscriber = Mage::getModel('newsletter/subscriber')->loadByEmail($email);
if ($subscriber->getId()) {
     // put your logic here...
}

Or if you have customer ID then you can directly check in newsletter_subscriber table to check if customer ID exists or not.

You have to check also the subscription status:

  if(Mage::getSingleton('customer/session')->isLoggedIn()){
        $email = Mage::getSingleton('customer/session')->getCustomer()->getData('email');
        $subscriber = Mage::getModel('newsletter/subscriber')->loadByEmail($email);
        if($subscriber->getId())
        {
            $isSubscribed = $subscriber->getData('subscriber_status') == Mage_Newsletter_Model_Subscriber::STATUS_SUBSCRIBED;
        }
    }

In my opinion none of the above answers are very convenient in that they either don't check if the customer is actually subscribed or don't handle the case where a subscription has not been found, so here goes:

$customerIsSubscribed = false;
$customer = Mage::getSingleton('customer/session')->getCustomer();
if ($customer) {
    $customerEmail = $customer->getEmail();
    $subscriber = Mage::getModel('newsletter/subscriber')->loadByEmail($customerEmail);
    if ($subscriber) {
        $customerIsSubscribed = $subscriber->isSubscribed();
    }
}