Check that nmbd is running:

ps -eaf | grep nmbd

Result:

root       3422      1  0 10:30 ?        00:00:00 /usr/sbin/nmbd -D
root       5178   4327  0 11:14 pts/0    00:00:00 grep --color=auto nmbd

If nmbd does not appear in the list reported by ps, it should be started as root using the normal means (/usr/local/samba/bin/nmbd -D).

Samba servers have a special NetBIOS name, __SAMBA__, to which they will always respond. By querying the server for this name, we can verify that nmbd is working correctly. The -U flag is used to specify the address to which the query should be sent.

nmblookup -U 127.0.0.1 __SAMBA__

Result:

querying __SAMBA__ on 127.0.0.1
192.168.0.2 __SAMBA__<00>
192.168.1.2 __SAMBA__<00>

If nmbd is not running, the following error message will be shown:

name_query failed to find name __SAMBA__

This can also be caused by not including the loopback interface name for the interfaces parameter in smb.conf and setting bind interfaces only = yes