They can't tell if you are a virgin or not. It used to be that a gyno could tell by checking to see if a girls hymen
was still intact or if it was torn. However, now we know that a girl can tear her hymen playing sports, or inserting a tampon. Therefore, the gyno can 100% tell if your hymen has been torn or not, but they can not decide whether it was torn through sex or sports/tampon insertion.
Hope this helps. :-)