It's been a couple years since I've taken a syntax class, and I've forgotten - what tests can you use to check whether a speaker uses a word as a conjunction? I seem to remember something about testing where the speaker can attach different clauses in the sentence, but I don't remember the details. I'd appreciate any explanations, or links to published papers discussing this topic!
In The Syntactic Phenomena of English, McCawley uses Ross's CSC (Coordinate Structure Constraint) and RNR as diagnostics for coordinate conjunctions. See, e.g., p. 616, where M. investigates contrastive negative but.