C:\>repadmin /showreps
domain_controller
OR
You can use Replmon.exe for the same purpose.
OR
AD Sites and Services and nslookup gc._msdcs.
To find the GC from the command line you can try using DSQUERY command.
dsquery server -isgc to find all the gc's in the forest
you can try dsquery server -forest -isgc.

The reason that all DCs are not GCs to start is that in large (or even Giant) forests the DCs would all have to hold a reference to every object in the entire forest which could be quite large and quite a replication burden.
For a few hundred, or a few thousand users even, this not likely to matter unless you have really poor WAN lines.

Active Directory® Service Interfaces Editor (ADSI Edit) is a Lightweight Directory Access Protocol (LDAP) editor that you can use to manage objects and attributes in Active Directory Domain Services (AD DS). ADSI Edit (adsiedit.msc) provides a view of every object and attribute in an Active Directory forest. You can use ADSI Edit to query, view, and edit attributes that are not exposed through other AD DS Microsoft Management Console (MMC) snap-ins: Active Directory Users and Computers, Active Directory Sites and Services, Active Directory Domains and Trusts, and Active Directory Schema.

NETDOM is a command-line tool that allows management of Windows domains and trust relationships. It is used for batch management of trusts, joining computers to domains, verifying trusts, and secure channels.

Repadmin.exe command-line tool assists administrators in diagnosing replication problems between Windows domain controllers. You can use Repadmin to view the replication status of each domain controller. In addition, Repadmin can be used to manually create the replication to force replication events between domain controllers, and to view both the replication metadata and up-to-dateness vectors. Repadmin.exe can also be used for monitoring the relative health of an Active Directory forest.