Drastic changes in your partner's physical appearance could indicate that your partner is trying to impress a new love interest. Changes could also mean that he or she is actively looking for another partner.  Look to see if your partner changes or replaces their wardrobe. For example, if your partner normally dresses in athletic clothing, but suddenly begins to wear suits or tailored clothing, this may be a sign of infidelity, especially if there haven't been any major lifestyle changes, such as a promotion at work. Your partner may join a gym or begin working out more often to tone their body or lose weight for their new interest. Your partner’s new object of interest could perhaps work or work out at the gym that he or she has been frequenting. A sudden focus on their physical appearance and concern with looking good are potential signs that there is someone else for your partner to impress. Your partner may start paying more attention to grooming if they are trying to appear attractive to somebody new. While in our current era it has become increasingly socially acceptable for both women and men to focus on personal grooming, a sudden change in habits could be a red flag.  Watch to see if your partner bathes more often, flosses their teeth more regularly, shaves more often, and other types of grooming activities. Be aware of new makeup, perfumes or colognes, either applied by your partner or as residue from another person. The old cliché about finding lipstick on a cheating man’s collar has endured for a reason. Observe whether your partner has changed their hair. Do they suddenly have a new look or start dying their hair a different color?

TL;DR in Vietnamese:
Để ý những thay đổi bên ngoài. Quan sát thói quen ăn diện của đối phương.