Why do you say she is definitely a prostitute?
Because that's what "they" say in all the art texts. She is also looking at the viewer directly, as if saying how much she charges. She's got the flowers from an admirer (so she must be good). She also has a dulled expression on her face; is not modest; and has "a working class body" (or so "they" say).
Prostitution was up and coming in the latter part of the nineteenth century, the Victorian age. Sex in the marriage bed was for procreation only. Men went elsewhere for "fun." And women were said to be shocked by this painting, and others by Manet, which highlighted prostitution.
Nudes prior to Olympia were either muses or portrayed as coy. In Olympia the woman is staring directly at the viewer. She is not shy at all, and is a prostitute.