Most orthodox Jews (especially women) can spot another frum woman a mile away. I can always pick out a frum Jew on the subway with a glance, unless the person is not dressed tzanua at all. I think most people here are debating on the “borderline” not the people who completely discount tznius…
Although a friend of mine relayed this story to me: At Columbia, the kosher cafeteria is in the Barnard area. The Jewish guys all go there for food. Once, a friend of hers started talking to a nice woman on line. She was wearing a long skirt and sweater set. They sat down to eat their meal together, and she took off her cardigan. He was shocked until he realized she wasn’t Jewish – just someone who decided to eat some of the kosher food that day. I thought it was kind of funny 🙂