It is not possible to know if an outgoing message has been read by your recipient.
This is because your recipient's carrier usually use a different system that may not communicate the message read status to our platform. Due to the nature of SMS text messages, we have limited information on whether your message has been read.
If you do not see an error message when sending your message, it is most likely that your message has been delivered successfully to your recipient's device. See Are my messages delivered?