There are few ways in which you can check the verison of the MOSS installed on the machine. They are -
1> Go to Central Administration Page. Click on Operations tab. Then click on "Servers in the farm" link. There the version should be available.
2> Its also visible in the site settings page of the Central Administration i.e. http://centraladminURL:portnumber/_layouts/settings.aspx.
3> Also you can run the below query on the Config database.
WHERE VersionId = '00000000-0000-0000-0000-000000000000'
ORDER BY [Id]