My question back would be Male or Female?
In males, I know no way to prove virginity.
In females, a virgin will have slight spotting at first sex. There are rare cases where the skin (Hymen) is broken due to an accident or for medical purposes. In older days, Priests would check the wedding bed after newly wed couples had sex to look for traces of blood. If there were, the wedding was blessed and if no trace, there would be trouble for the woman. In bible times, she could be stoned and her family disgrased.