a mile away
Very easily. Always of something detected or observed. I can spot Paul from a mile away, just by the way he walks. After decades as an investigator, I can identify a con man a mile away.
miles away
Noticeably distracted, unfocused, or lost in thought. Of course you don't remember me saying that—you were a miles away the whole time! Are you even listening to me? You seem a miles away.
miles away
If someone is miles away, they are completely unaware of what is happening or of what someone is saying, because they are thinking deeply about something else. Sorry, I was miles away. Could you repeat that? You didn't hear a word I said, did you? You were miles away.