Your friends don't know what they are talking about.
The only clue to her virginity is whether or not her hymen is intact. But a girl who is a virgin can accidentally break her hymen through non-sexual contact (I broke mine when I landed on the cross bar of a men's bicycle) and non-virgin can have her hymen surgically repaired. So, this certainly isn't a 100% reliable test.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.