The following exchange appears in Jonathan Lethem’s novel Girl in Landscape (on p. 208 of my Faber and Faber edition, 2002):
“I don’t have a home,” said Ben Barth.
“Well, who’s fault is that?” said Wa.
Who’s is a contraction of who is or who has (or occasionally who was): Who’s going? Who’s got tickets? Looks who’s talking; whereas whose is a possessive pronoun – it’s who in the genitive case – so it should have been used in the quoted passage: whose fault is that?
Confusion arises because who’s and whose are pronounced identically, and also because the ’s in who’s can mislead people into thinking it has to do with possession: If the cap isn’t Jo‘s or Jim‘s, then
who‘s whose is it? (This apostrophe-led impression of possession probably also inspires the erroneous your’s, her’s, our’s and their’s.)
Who’s for whose is a common mistake in informal writing, and it sometimes sneaks past editors too. To keep who’s in its rightful place, you can use the same mnemonic I recommended for it’s and its: just as it’s always means it is or it has, so who’s means who is or who has. Bring this to mind any time you’re uncertain, and you shouldn’t slip up.
I liked Girl in Landscape, incidentally; it’s a coming-of-age story in a sci-fi setting with elements of mystery and western. It also has examples of dialectal would of (We should of killed them; you’d of met him), which I wrote about recently. I’m not a fan of the construction, but since I’ve seen it in dialogue from several capable authors, I’m inclined to give them the benefit of the doubt. But I can’t say the same for who’s fault.