You want to find the PDC Emulator for a domain using DNS.
> nslookup -type=SRV _ldap._tcp.pdc._msdcs.<DomainDNSName>
The PDC Emulator FSMO role is the only FSMO role that is stored in DNS. Like many of the other Active Directory-related DNS records, the PDC record is stored as an SRV record under _ldap._tcp.pdc._msdcs.<DomainDNSName> where <DomainDNSName> is the domain the PDC is in.
Recipe 3.21 for finding domain controllers via DNS