When a Direct message exchange occurs successfully, the sender knows for certain that the message reached the recipient as each sent message is replied to with a MDN (message delivery notification). The MDN informs the sender that the exchange was successful, but not when the recipient read the message.
MDNs are managed by the HISPs and thus are guaranteed to be sent unlike return / read receipts in regular email which are reliant on the mail client and end user respecting the senders request to be informed when the recipient reads the message.