And if more than one are in use, how does anyone know which one is
pertinent?

Who declares the "active" at an uncontrolled field?

Good question. Just today I was about to take runway 19 at an
uncontrolled field when
another aircraft announced he was turning final for 1. The wind, such
as it was, was a slight breeze from the South - the windsock was limp.
So it was up to the pilot's diecretion as to which to use. I held short
while the other aircraft landed (and declared "clear of the active")
before taking the runway. No sooner had I done so than along comes
somebody else on an IFR approach for 1.

This brings up an interesting question: Who has the right of way? Me,
because I am already on the runway, ready to take off - or the traffic
on approach, because it was "landing" (though had not so stated - only
an "IFR Approach" called out). As it
turned out, I held at the departure end, and the traffic declared a
missed approach (perhaps having intended to do so all along?), and
passed overhead.

Bottom line: Its a crapshoot. Anybody remember the old Tetrahedrons
that were used to indicate the active runway in the old days? I assume
that they went away because they
1. required someone to decide which way to point them, then 2. go out
and turn the Tetrahedron. That, of course, requires an employee on duty
- an endangered species in this penny-pinching age.