You'll have to parse it out of either the dn (distinguishedName) or the AdsDSPath attributes. Domain name entities are prefixed with "DC=" in these attributes. The leftmost DC= will contain your netbios domain name.

For example: cn=myuser,ou=users,dc=mydomain,dc=mycompany,dc=com

mydomain is the netbios domain name.

You'll have to parse it out of either the dn (distinguishedName) or the AdsDSPath attributes. Domain name entities are prefixed with "DC=" in these attributes. The leftmost DC= will contain your netbios domain name.

For example: cn=myuser,ou=users,dc=mydomain,dc=mycompany,dc=com

mydomain is the netbios domain name.

EDIT:
As Brian Desmond points out, this is not necessarily the authoritative way to find the actual netbios name, it is merely coincidence that they usually correlate. See BoyMars' answer for the authoritative way.