She might start to be less available for family time. For example, she often makes excuses to be out alone or with 'friends' more frequently. Also, a change in her sexual behavior with her partner can be a sign. If she suddenly seems disinterested or overly eager to try new things that are out of character, it could be a red flag.
One sign could be a change in her appearance. Maybe she starts dressing more provocatively or taking more care with her looks all of a sudden. Another sign is if she becomes overly secretive about her phone. She might be constantly on it, hiding texts or calls, and getting defensive if someone asks about it.
Well, she may start to distance herself emotionally from her family. She could be less affectionate towards her partner and children. Her mood might also change, becoming more irritable or withdrawn at home. Additionally, if she starts having unexplained expenses, like extra money spent on gifts or dinners that she can't account for, it could be a sign that she's involved with someone else.