True, these questions can be difficult -- unless the writer only treats writing as a hobby, perhaps (e.g. by relying on some other line of work to bring in a regular salary, while trying out writing as an evening/weekend job etc). Some writers start out like this.
for instance, before Terry Pratchett became a successful writer, he worked for many years as a journalist for a local paper (the "Bucks Free Press" in Buckinghamshire), and then as a press officer for the local board that supervised nuclear power plants. He wrote three sci-fi books in the 70s and published them while working there.
Two years after starting his job at the energy board, he released his first "Discworld" book ("The Colour of Magic", 1983). His popularity (and sales) increased dramatically over the next 4 years, and in 1987 he became a full-time writer. The rest, as they say, is history.
Of course, for every Terry Pratchett there are thousands who try ...