In "The Invisible Japanese Gentlemen," how can one tell that the girl in the story is a twentieth-century character?

There are several cultural and other clues about the time in which this story is set. Most tellingly, the young woman in the story seems content to make her own way in life and to pay the way for her marriage. She informs her fiancé that the advance on her book will enable them to be wed, and she snaps up the bill for their restaurant meal and insists on paying it. She refers to her fiancé as "awfully decorative" in a dismissive way. All these behaviors show that she is a woman of the women's-rights era.
In addition, the author says of the young woman novelist, "I didn't like to think of her as the Mrs. Humphrey Ward of her generation." Mrs. Humphrey Ward was a British novelist who lived from 1851 to 1920, so the young woman in the story has to come after this time, as she is the Mrs. Humphrey Ward of a newer generation.

There are many clues that suggest that the story is taking place in the twentieth century.  Greene's narrator discloses the setting as London, and the younger characters are drinking Chablis in a restaurant.  There is a group of Japanese businessmen there as well—an unlikely scenario prior to the twentieth century. The narrator listens to the girl's "harsh" way of speaking and concludes that she has recently left university.  Instead of a traditional engagement ring, she wears a man's signet ring on her finger. These details suggest a modern setting.
The conversation that the young woman and her fiance are having indicates that she is a fiction writer with a publisher; she is openly advocating for them to marry right away since she has received a financial advance on her book and can support them.  This marks her as a liberated woman who has a career and is unafraid to ask for what she wants. 
When they are getting ready to leave the restaurant, she insists on paying the bill since the celebration is because of her upcoming novel, The Chelsea Set.