Well I don't think she has, for definate, but she was suspicious when she saw Rosemary stirring perfectly good orange juice with a spoon (and so would I be). She took the offending orange juice to the police, at which point, unbeknown to her, Rosemary's inspector-friend stepped in and quietly advised the investigating officer that poor old Purdy had been frightfully poorly and it might be better if he let the whole thing drop.
Of course Purdy doesn't know that - but whether the juice had already gone to the lab I don't think we're supposed to know at present. Certainly it's going to be difficult for Purdy to prove anything with that obsequious inspector about (what is he, by the way, Chief Inspector or Chief Constable or something, I've missed that), so she's going to have to find hard evidence before 'mummy's boy' Gray will listen!