There are no outward signs of a person's virginity, or lack there of, although a common physical change for women comes in the form of a broken hymen membrane. This is not a foolproof sign because the hymen can be broken when falling or with other acts unrelated to vaginal penetration.
Identifying whether or not a person is a virgin is made all the more difficult due to the varying definitions for the term. Some people believe that only vaginal penetration results in a loss of virginity, while other forms of sexual activity do not. With no concrete definition, and no signs that mark a person as different to the casual observer, only behavior can be used to judge whether or not a person is still a virgin.
Every person is different, and where some may demonstrate obvious changes in manner or demeanor, others may not. Experts at Planned Parenthood encourage young adults to focus on how they feel about their sexual experiences rather than whether or not they are considered to be a virgin afterward. In the end, all that matters is the relationship between the two people involved in the sexual experience and their feelings about it later on.