Is he always in a good mood for no reason suddenly? Smiling myseteriously? Do you overhear him talking to someone in a different way than normally.does he have converations where he is only listening and not saying anything?
He starts acting differently, your conversations are shorter,he has "i don't care altitude" toward your relationship,he doesn't laugh at ur jokes anymore.he seem to lose interest in fun actitives you guys share together.
When he is engaged in conversation with another human being*.
*(unless he can speak to dolphins or thinks he can speak with other animals)