Perhaps you can fiddle with the nis server start script so ypserv is run with an additional -v argument, and ensure (touch) a file /var/yp/ypserv.log
But it is poorly documented, and I never tried it.
I remember I used a poorly documented snoop command. (And have found it here)
snoop rpc ypprog
BTW other useful snoop commands are
snoop rpc nfs and
snoop port domain (for DNS traffic)