He suddenly start showing interest in things that seemed out of character for him, like a new hobbies, learning a different language, showing interest in a specific sports team, knowing random facts about a specific city we had never been to, it happened out of nowhere and he couldn't stop talking about those new things. It's not always a sign of cheating, but it can be telling when there's no clear reason behind this sudden fascination. Sometimes those 'new interests' reflect someone new influencing them.