Suppose if there are two user accounts with the same name (vprabhu on
local (i.e. files) as well as NIS), then /etc/nsswitch.conf determines
which domain to authenticate against. However, depending on the OS
(for example authconfig settings in linux) can alter the nsswitch.conf

For example,

cat /etc/nsswitch.conf|grep passwd
passwd: nis files

then if vprabhu logs in it will be authenticated against NIS. However,
if authconfig settings are "Local authorization is sufficient" is ON,
it will authenticate against FILES.

Now, given this situation, how do we reliably know against which
domain (local/NIS) a user has authenticated against while logging in ?
If there is a POSIX API or portable API or even OS commands across
major UNIX versions please let us know.