Here's how I judge when someone's playing a devil's advocate and isn't really interested in the discussion, but rather enjoying "playing" with the people.
Leading questions --- Usually answering questions with questions designed to poke and prod at the person's mind in an effort to gauge the person's stance before actually revealing their own point of view
Tone of voice --- semi-curious, either very confident, or slightly tentative, also slight hints of sarcasm designed to provoke some sort of emotional response in order to cloud the other person's rationality
Posture --- Overly calm, unemotional which shows dispassion about the subject. Leaning back, or forward - the gaze is either too piercing or almost distracted. Sometimes busy playing with something. Not really using hand gestures a lot. Not very fidgety and usually very controlled body language
Facial expressions -- Smirks slightly at responses that the devil's advocate can easily counter. A gleam in the eye that one gets when they've "cornered" someone. A wry smile when the other person starts getting emotional.
Verbalization of arguments --- rarely able to hold a consistent view-point, jumping from argument to argument, usually contradicting self, and ignoring call outs of inconsistent thought. Arguments laced with subtle personal attacks. Almost always has absolutist statements. Tries to go in for the kill slowly. Most often doesn't reveal their own stand and able to switch from one side to the other depending on what view point the opponent is taking.
So .. how do you read when you're being "played" in an argument?