@Roberta,
The expression (pig in a poke) is common to English speakers about anywhere, i'd assume, from its currency in North America, which despite the narrow vision of other English speakers, is not a monolith of a single idiom, or even of a handful of them. People in one part of a state or province will comment upon the strange speech of those from another part of the same state or province. In Illinois, people from the Chicogo area will often say "Saint Louie," while in the central part of the state, they will say "Saint Louis," and in southern Illinois, many of the native born say "Sant Louis."
It can also be observed that although people all over the country may use "wicked" as an intensifier, it frequent use in New England makes it noticeable as a regionalism. Some people will say sack rather than poke, while others will say gunny sack rather than either.
All in all, i'd say FM has a good point about regionalisms being a useful forensic tool. You would expect the dispute between sack, poke and gunny sack among countrymen--not so much among city dwellers. The use of particular words, and the pronunciation of them, can be clues to more narrowly identify someone. Arthur Conan Doyle's medical school mentor, Dr. Joseph Bell, was said to have been able to identify the exact neighborhood from which a patient came by their speech, and their profession from their appearance. I have no reason to assume that a veteran investigator today would be any less clever.
