in the days of the early Christians, they used the sign of a fish. Perhaps now is the time for the JEWS to come up with a secret sign of their own, find one soon. God Bless Israel.
There is body language, language, accent, catch phrases, attitude. It is about as hard for an Arab to pass as a Jew as it is for a Jew to pass as an Arab. The cultures are similar, but too different to easily fake.