Am I able to check who has read published messages?

It is possible to implement a system to check who has read messages, such as having subscribers publish when a message is considered received, however, it is not natively supported.

Pub/Sub is designed such that the publisher shouldn't know or care how many recipients there are.  As such, it is difficult for us to work out who has read/seen/received a message, as we're not in control of how many clients are expected on that channel.