Extension that shows synchronized invoices and creditmemos by Advisie / Exact Online in Magento 1.8.x / 1.9.x
Download the zip (or clone), install in Magento 1.8.x or 1.9.x and clear the cache. In the admin, navigate to Sales -> Invoices and all synchronized invoices are marked by a green check. Non-synchronized invoices are marked by a red mark. Same goes for the Credit memos. Make sure your cron schedule runs! The cron job for the checking runs every half an hour.