In this interview in the Forward, those Hasidic guys who bug people on the streets of New York asking if they’re Jewish explain how they know who’s Jewish and who’s not:

First, said Yisroel, “we call it ‘racial profiling.’ Who looks Jewish?” (When asked to clarify later, Yisroel says it’s not about the nose — a “broad, clear forehead with no creases” indicates a non-Jew, while Jews’ foreheads are sometimes lined.) Next is detecting a subtle vibe of recognition, a process that Levi calls “bageling.” Third is playing the statistics game. One out of every five people in New York City are Jewish, said Yisroel. If you exclude African Americans and Asians, your odds are closer to one in three.

I’m not sure what “bageling” is, but I claim to have a very highly-attuned Jewdar. Mostly, if a girl is wearing long sleeves under a short sleeve shirt in the summer, or a group of non-athletic guys is wearing baseball caps, they’re probably Jewish. Or, if a guy looks anything like me, he’s probably Jewish.