So, by next Sunday, Jane should know who Red John is, right...  Think about it – Jane is highly intelligent
and has the entire weight of the CBI and their databases behind him. He has a
list of approximately 400 suspects.  In
addition to all the other things he knows about Red John, he now knows that Red
John was on that farm on particular day (the date on which that woman shot the
one guy) in 1987.  It wouldn't take long
to eliminate most of the list.  The
majority of his suspects will have alibis for that time/place.  (As in they were too young, too old, living
in another state, etc.) The one suspect that has no alibi for that date AND no
alibi for the dates/times of the other Red John murders is Red John. Seems
simple enough.