The folks who have been doing it for a while are able to recognize male/female by looking at the plastron (bottom shell), anal scutes (part of the bottom shell right above the tail), and the tail. I have gotten pretty good at i.d.ing the kinds of tortoises that I personally come into contact with, however, when I recently took in a Greek tortoise (not one I usually come into contact with) I thought the tail looked male. But when I posted a picture of it here, I was told it was a female. I was going by how far away from the body the cloaca (vent) was. Usually the male's cloaca is further away from the body than the female's. However in this instance if I could have seen a male and female side-by-side I would have seen that the male's tail is much bigger than the female's.
I hope that helps. However, in the meantime, while you are learning, send us a picture of the bottom side of the tortoise, including the tail, and someone here will let you know what you have.