Okay, in the episode where Carter and O'Neill get sent to the Stargate in Antarctica, they do not recognize that it is a Stargate on Earth for the very simple fact that the point of origin symbol on the gate is not the Pyramid+Sun, but instead some very complex-looking symbol. The main point is that it is NOT the well-known symbol of the show. However, after the Stargate in the SGC is teleported up onto Thor's ship, and the ship crashes, and the Beta Gate (from Antarctica) is substituted in the SGC, oddly enough the symbol for point of origin is now the Pyramid+Sun, not the weird complex symbol. If it had been that way originally, Carter and O'Neill would have known they were in Antarctica that one time.