If you are a boxer and you want the audience to know you're Jewish, how do you make it obvious?

You can wear the Star of David on your trunks. Max Baer did that and became World Heavyweight Champion. Alas, from time to time, goyim appropriate the Magen David. James Toney, world champion in three different classes, wore one, and he was pure goy. (It was a tribute to his manager?)

Jack "Kid" Berg, World Welterweight Champion in 1930, did wear a star on his trunks. But he also wore tefillin! (For the goyim: tefillin are prayer boxes worn by religious Jews, one on their forehead, and the other on the arm.) No, these boxes are not for boxing. We assume he took them off during the bout?

Another way? Well, you can get a Jewy nickname... say the old standby, "The Hebrew Hammer". Berg eschewed that; other than "Kid", he was also known as "The Whitechapel Windmill".

Whitechapel? Perhaps he wore the tefillin to compensate?