Magento – Display stock status on product detail page

Just set Manage Stock to Off.

To display the "out of stock" and "in stock" label on product view page, write the below code in the file app/design/frontend/rwd/default/template/catalog/product/view.phtml

<?php    
$stock = Mage::getModel('cataloginventory/stock_item')->loadByProduct($_product);
    $qty = $stock->getQty();
    if($qty <= 0)
    {
?>
       <span class="out-of-stock"><span>Sold Out</span></span>
<?php
    }
    else
    {
?>
       <span class="in-stock"><span>In stock</span></span>
<?php
    }
?>

And if you want the label on the category page, write in app/design/frontend/rwd/default/template/catalog/product/list.phtml

NOTE : It is assumed that you are using magento-1.9 or greater