Only by the appearance of the hymen or the lack thereof. Be advised that the hymen can be broken by ways other than sexual intercourse such as riding a horse or falling off of a bicycle so this method is not 100% efficient.
By picture one can not distinguish. Some people mistakenly think that one can distinguish via physical exam someone who has had sex from someone who has not, but that is not true.
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....
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.