malcolmdl:
Plausible means?
Gossip -- husband, daughters, servants, friends in the 'psychic circles' of the times, overheard conversations of servants who were 'invisible' to the upper/middle classes of this time. (People discuss the strangest, most personal things in front of bartenders even nowadays -- I can attest to this)
Paid Informants. (Mrs Piper earned a living at this game -- plenty to go around, I think) Servants could easily be employed for "a small consideration, Ma'am".
Newspapers -- "hatches, matches and dispatchies". Local newspapers also report on things like deaths from accidents.
Register of Births, Deaths and Marriages.
Local Biographical Encyclopedias that contained a wealth of information.
The "Who's Who", if these existed then. Or maybe the "Woos Woo"
Compromised post -- getting a hold of mail send to and by folk like James, Lodge, and Hodgson before they got it, opening and resealing, then having it delivered as unopened. 'Reading through the envelope' is easy, too. Here is a method that would 'reveal' some of the supposedly annonymous sitters. I assume most were contacted and made appointments by letter. There was usually only one or two new sitters per day. So, if Mrs Piper knows that Sir Oliver Lodge has made an appointment for Mr X on Thursday, 14 Dec. 1894, she knows who Mr X is and has time to research. The postboy or even postmaster could be involved. Remember that in the past telephone exchange girls were often guilty of 'listening in'.
Overhearing via the plumbing, by holding a glass to wall or floor or by 'cowans and eavesdroppers' outside windows -- not as silly as at first these methods might seem.
The 'psychic blue book system'.
Her own Private Investigators.
Sneaking about in the homes she was in, by her or a confederate. Even locked doors and cabinets can be opened, after all.
Collusion with investigators, witting or unwitting. They, too, made money from the Piper 'franchise.' People were paid, books were published, lectures given, etc. Hodgson was sent to America -- not a bad little number for him IMO.
Mrs Piper did fish for info -- all the investigators admit this. So, a mixture of cold/hot reading gives varied performances which is just what we would expect. She is 'fluent' when she has the info she needs, and fishes when stumped.
A mixture of all or some of the above.
I'm sure there are other explanations that don't involve the paranormal.
On the pre-reading of the mail, the following is most suspicious: