You cannot (just by looking). A rabbi generally dresses like any other observant Jew.
Rabbinical student
Elie lies to Rabbi Eliahou about not seeing his son during their march to the concentration camps. Elie witnesses Rabbi Eliahou's son distancing himself from his father during the long and arduous journey, and he chooses not to share this information with Rabbi Eliahou out of fear.
A guy, a rabbi, and Jesus walk into a bar. The bartender looks up and says,"Don't tell me this is some kinda stupid joke"
This is quite an interestting question and one about which I too have wondered - enough so that I've asked a few rabbis, and none of them has been able to tell me definitively that being Jewish is an essential requirement in a rabbi (and a question a rabbi cannot answer is a rare question indeed). A rabbi is, after all, a teacher rather than a priest; and thus it seems that, should someone who is not Jewish succeed in persuading a yeshiva to accept them and later obtained semicha (rabbinic ordination) there's no reason that he or she should not be considered a rabbi. However, finding a yeshiva open to non-Jewish rabbinic students would be in all likelihood all but impossible, and even the most liberal of Liberal Judaism congregations might have trouble accepting a non-Jewish rabbi.
Originally, it was a rabbi, a rabbi, and a rabbi walk into a bar. . .
Rabbis. Here are a few examples from the Talmud. Rabbi Yochanan ben Zakkai, Rabbi Yonatan ben Uziel, Chanina ben Dosa, Bava ben Buta, Shimon ben Hillel, Rabbi Eliezer, Rabbi Yehoshua, Rabbi Yossi haKohen, Rabbi Shimon ben Netanel, Rabbi Elazar ben Arakh, Rabbi Yochanan ben Nuri, Rabbi Akiva, Rabbi Nechuniah, Rabbi Nachum Gamzu, Rabbi Yossi Glili, Rabbi Honi Me'agel, Rabbi Abba Shaul, and hundreds of others. Each of these had large groups of disciples.
Rabbi Aaron Teitelbaum and Rabbi Zalman Leib Teitelbaum
A rabbi is a teacher.
A Chief Rabbi
The rabbi of France
Eleizer forgot to tell Rabbit Eliahou that he indeed hadseen his son, and he noticed that he left him behind on purpose.
how can you tell if someone has moved on