She pays increased attention to her appearance
Women involved in new relationships often become preoccupied with their looks.
According to AskMen.com, it is a sign that your girlfriend is cheating when she suddenly changes hairstyles, starts wearing a new perfume, buys new clothes (particularly lingerie) and starts working out more at the gym.
Assuming that she hasn't started a new job or had some other big change in her life, take these changes as signs.