How can you tell if someone is a Freemason without asking them directly?

Usually they are members of a Freemason club, which most cities have, so you can find out who the members are. Sometimes it operates under something like the Rotary or the Lions Club, and those members are Freemasons. There may be hand gestures as well, but simply being a member of a Masonic institution is a sufficient tell, and that is fairly easy to find out.