If she had penetrating vaginal sex, she has lost her virginity.
It's not possible to just look at anyone and know whether or not they have had sex before. You can make an educated guess, but that does not mean you'll be correct on the matter.
You cannot and will not unless she tells you, you've had sexual intercourse with her, or you somehow witnessed her having sexual intercourse.
He knows when he has had sexual intercourse for the first time, and is conscious of it.
If she's consciously engaged in sexual intercourse, then she'll know she has lost her virginity.
There is no definitive way to know except to ask him. He may or may not tell the truth, so it's up to the person asking to make a judgment call.