You're right; it is a tricky one! There are definitely quirks of style and things, but usually I'll only be relatively sure when it's someone whose work I know really well. (And even then that's with clues like, "this small fandom of two or three" and "Well, x always turns the hit count off on AO3"). I think I've detected a friend in the ChocoBox exchange, so when reveals come round I can tell if I'm any good at this or not...