They become more secretive about their phone and computer habits. This could include hiding their phone, changing their passwords, or using their phone more often in private.
They start flirting with other people. This could be overt, like making eye contact with someone across the room, or it could be more subtle, like making suggestive comments.Image: Google
They start making excuses for why they can't be home with you. This could include saying they have to work late, or that they have to go out of town for a weekend.
They become more withdrawn or distant. They might not seem as interested in talking to you or spending time with you.
They start lying to you about where they've been, who they've been with, or what they've been doing.