Because Barbara Ehrenreich approached Nickel and Dimed as a social experiment instead of a scientific one, her methodology allowed more conjecture and assumption. For example, she couldn't easily wear a microphone to record all her conversations, so most of her dialogue is transcribed from memory rather than from tape. It can be assumed that she took notes, but most of the book was written after the experiment took place, and so she had to reconstruct her experiences from fallible memory.
Perhaps the most obvious element of fiction are the names, which Ehrenreich changed for privacy; we have no way of knowing the real names or even genders of the people she names and talks with. However, this is standard technique when dealing with real people in non-fiction books.
Ehrenreich also approaches each section of the book like a short story; she sets up her dilemma, details her attempts to resolve it, and shows the conclusion where she inevitably fails. The characters, although never fully fleshed out, act as foils for her thoughts, so the book can be compared to a novel with Ehrenreich as protagonist. However, the masses of statistics and footnotes get in the way of her straightforward prose.
Finally, since Ehrenreich was playing a character to seek employment, the entire book could be seen as an elaborate role-playing game in which she lived and thought as her character, adjusting her speech and behavior to avoid detection. Strictly speaking, this is not an element of fiction, but the act of creating a character utilized those elements for believability, and Ehrenreich succeeded in creating a version of herself "In [a] parallel universe where [her] father never got out of the mines and [she] never got through college (Ehrenreich, Nickel and Dimed).