Today I am surprised to learn that 俞大絪, an authoress of an English textbook, is known as a 先生 (and it is very unlikely a Mr. as the word indicates because she is a real her). After searching this great site, I have found some interesting answers in 先生 used as title for female.
But the question still remains incompletely answered. For example, what is the qualification of this title? How can one judge the gender of this title holder without being shown photo or ID card? How is she addressed in English, or is there an equivalent in English for this title?