How about this protocol:
You ask the person who you suspect to be a fader: "Do you know what a tat is?"
If the person knows ttatt, they then must respond with the question: "Is that tat spelled with 4 T's?"
(If it's a sleeping JW they'll just say: "No, I don't" or "Is that short for tattoo?" If the sleeping JW ask you why you ask you could just say you overheard someone talking about a tat and wondered what that was.)
Then you reply by saying: "Maybe. Do you know what it means?"
Then they must say: "Are you fully awake my brother [or sister], you look a bit drowsy?
Then you must say: "Oh yes! I'm to-ttatt-ly awake!"