Example snippet of helper class, that checks if the order items are owned by the current customer

order ids helper

<?php
/**
 * @author      Tsvetan Stoychev <tsvetan.stoychev@jarlssen.de>
 * @website     http://www.jarlssen.de
 */

class MyCompany_MyModule_Helper_Data extends Mage_Core_Helper_Abstract
{

    /** @var Varien_Db_Adapter_Interface $readConnection */
    protected $_readConnection;

    /**
     * @param array $itemIds
     * @return bool
     */
    public function canModifyOrderItems($itemIds)
    {
        $select = $this->_getReadConnection()->select()
            ->from(array('oi' => $this->_getResource()->getTableName('sales/order_item')), array('item_id'))
            ->where($this->_getReadConnection()->quoteInto('oi.item_id IN (?)', implode(',', $itemIds)));

        $select->joinRight(
            array('o' => $this->_getResource()->getTableName('sales/order'), array('customer_id')),
            $this->_getReadConnection()->quoteInto('oi.order_id = o.entity_id and o.customer_id != ?', $this->_getCustomerId()),
            array('customer_id')
        );

        $result = $this->_getReadConnection()->fetchRow($select);

        return empty($result) ? true : false;
    }

    /**
     * Getter for the read connection
     *
     * @return Varien_Db_Adapter_Interface
     */
    protected function _getReadConnection()
    {
        if(null === $this->_readConnection) {
            $this->_readConnection = $this->_getResource()->getConnection(Mage_Core_Model_Resource::DEFAULT_READ_RESOURCE);
        }

        return $this->_readConnection;
    }

    /**
     * Getter of the resource model
     *
     * @return Mage_Core_Model_Resource
     */
    protected function _getResource()
    {
        return Mage::getSingleton('core/resource');
    }

    protected function _getCustomerId()
    {
        return (int) $this->_getSession()->getCustomerId();
    }

    /**
     * Get checkout session model instance
     *
     * @return Mage_Customer_Model_Session
     */
    protected function _getSession()
    {
        return Mage::getSingleton('customer/session');
    }

}