Everyone is/was a virgin at some point in their life. Once a person has had sexual intercourse, they are no longer a virgin and cannot go back to being one.
You can't tell. Some refer to the hymen but it breaks easily and some are born without one. 80% do not bleed during first intercourse or feel pain. It looks the same before and after intercourse.
You can't. People say an intact hymen indicates a virginity but this is untrue (or at least highly unreliable) as there are many common activities besides sex that can break the hymen.You can't tell....