To check the version of Samba that you are running, use the smbstatus command. The first line is the version, which should look something like:
Samba version 3.6.3
If you want to see just the version without the rest of the smbstatus information, you can run:
smbstatus –version
Much less cluttered. The above is the current version for a 12.04 Ubuntu server. Still waiting for version 4!