If all that was visible were the various vote tallies and numerous
voters were casting their ballots simultaneously, how would they know
for sure?

In the smaller districts and towns, somebody observant enough could
probably figure out how each person voted. There are many cases where
votes are non-simultaneous. I've done something similar myself in a
different environment based simply on precise statistics after the fact;
doing it live would be trivial.

Jose
--
You can choose whom to befriend, but you cannot choose whom to love.
for Email, make the obvious change in the address.