No-one, not even a gyn, can tell if you've had penis-in-vagina sex (or if you inserted other objects in your vagina for that matter), unless it was very recent and you have sperm in/on you, or vaginal tears.
This question and other common ones are answered by Scarleteen here: http://www.scarleteen.com/boards/faq.html

I guess a current STI with genital symptoms would also be a giveaway. But apart from that, no.

But dreamydancediva is right that it's good to share all information with your gyn so that s/he can treat you properly and give you STI/STD screens. No doctor can help you properly if you don't tell them your history.

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Londongirl
University Student

Unless your health is really seriously at risk, they are not allowed to say anything.

Just be honest, everything will be fine-I promise..they are there to help you and offer you support, not to tattle tale on things that you do...

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*~*Scarleteen Advocate*~*

Hearts will never be practical until they are made unbreakable

It states that they will encourage you to find a way to talk to your parents, but that they will respect your confidentiality if you don't want to do that. So that's at least one place you could go if you really can't face talking to your parents. See
http://www.plannedparenthood.org/about/thisispp/mission.html#07Adolescent

I suspect other doctors may well provide confidentiality for you too, e.g. your own doctor. If you are in doubt, why not ring up the place you want to go to and ask what their confidentiality policy is for people of your age, or discuss it with the doctor when you arrive. You have a right to know.

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Londongirl
University Student