Well, he was definitely a sexist.
There was one story where he spotted a male criminal in drag, because even though he was dressed as a woman, he was sitting in a restaurant with his back to the window.
According to EC, everyone knows that the woman sits facing the window, because she has better table manners than the man.
I think at this point I dropped Encyclopedia Brown and just stuck with Slylock Fox.
edit: On second thought, it was probably his sidekick Sally who figured it out. She'd get to bust out her Distaff Detective Skillz every now and then on particularly girly subjects.