All invoices can be validated in terms of distribution status and where it has been sent.
Go to Invoices under the "Sales" Heading. Invoices with status "sent" have been delivered to the recipients specified e-mail/Peppol ID. Qvalia will contact the sender if an invoice cannot be delivered.
To see where the invoice has been sent and if it has been sent via Peppol or e-mail, go to the actual invoice and press "Timeline".