It just means the sms client can tell the difference. How exactly could be anything. They don't know enough to even know that this would require two seperate back-end services rather than the same back-end service with some optional extra special data added to the transaction. They don't even think of the words "client" or "back-end" or "transaction" like I just said. The fact that their iphone can tell that the other phone is not using imessage is not remarkable at all to them, or me for that matter.