How can one know that which DNS ip is the primary or secondary from
This is not a DNS server concept -- usually irrelevant to the
client even if it were.
DNS servers use Primary (one that changes the records, i.e., the master
copy) and Secondaries (additional DNS servers that copy the zone file
from another master.)
Clients NIC-IP properties use "Preferred" and "Alternat" but this isn't
that big a deal because the clients try to find the most responsive -- best
is to have the NEAREST/fastest server as preferred.