The short answer is: you don’t — unless they choose to tell you.
Freemasonry does not issue membership cards that must be displayed. There are no lapel pins required by obligation, no public registries, no database you can search. A Mason may wear a ring, carry a token, or display the square and compasses on his vehicle — but he may just as easily wear nothing at all. The choice is his, and the silence is deliberate.
This discretion is not deception. It is the legacy of centuries during which Masons faced persecution, imprisonment, and worse for their affiliation. In 1738, Pope Clement XII issued the first papal bull against Freemasonry, and Catholic monarchies across Europe began hunting lodges. In 1826, the Morgan Affair in upstate New York triggered an anti-Masonic political movement that nearly destroyed the fraternity in America. Men learned to keep their membership close — not out of shame, but out of survival.
Today, the danger has passed, but the habit remains. A Mason reveals himself only when purpose demands it: to aid a Brother in distress, to verify a stranger in a foreign lodge, or to answer an honest inquiry from a worthy man. If a Mason does not come out openly, you can never know — and that is exactly as the Craft intends it.
Benjamin Franklin wore his Masonic affiliation openly, but George Washington was more reserved. Both were Grand Masters in their jurisdictions. Neither was less a Mason for his choice of silence or speech.