[girvo]

> by looking at how many devices iMessage is sending copies to when messaging the target

Is iMessage centralised? I'm pretty certain it is, and if that is the case then you couldn't find out if you were tapped or not; one message gets sent to the server (perhaps with a list of the devices you want to send it to) and the server under Apple/LEO control sends a copy to their "device".

[hwatson]

Each message is encrypted individually for each device that will be receiving the message. As a result, unless Apple slip a public key they have control over into the keys reported for the receiver, they cannot read your messages. (This is why abalone mentions that Apple do not have access to your old messages.)

http://blog.quarkslab.com/imessage-privacy.html goes into detail as to how the key exchange process works.