Is it possible to check if a specific message has been delivered to a device?

You can ensure the message was successfully published by checking the history of the channel for your message, however it is only possible to check if a device has received a message from the device itself. 

Ably does not store any per-message logs, nor logs of who is subscribed to a channel at any point in time. This means we are unable to check which users have received messages retroactively either.