I let a friend borrow my ipad that has some imessages saved. I signed out before lending it to him, but I got a notification that my ipad is signed into my imessages now. When I asked if they signed into it, they said their kid hit the button on accident, but if it's signed in now, will I get a notification if they are reading through my messages? Or is there a way I can find out?
You will see that the "unread" status of the message changes, if they're the first to read the message.
Apart from that you cannot really tell if they read the messages or not.