Can a doctor tell if i've been sexually active with a guy?

No: The answer is no, regardless of whether you're a guy or gal. So don't worry about your orientation or sexual history as far as we're concerned. However, as far as you're concerned, it's a good idea to talk to your doc & tell him/her what you've been up to since different activities have different risk factors. In other words, you need to be screened appropriately for various stds.

Not really: If you show up with a symptomatic sexually transmitted disease it would be evident you had an infected partner.Beyond that, there are no obvious signs of sexual activity.