What's the best approach for auditing a MOSS 2007 farm and ensuring it's correctly configured, which service accounts are in use.
However, it sounds that you want to audit the fact 'that the farm has been configured correctly'. You may want to try Microsoft's best practices Analyser for SharePoint.
Update: See this related question.
I've not found a free tool that does it all. There are some commercial products that with some customisation could be used but in the interim I've used these
This can give some of the information you need:
Provides a report of server roles, content databases and accounts etc.
And the SharePoint Administration Toolkit: http://blogs.technet.com/b/tothesharepoint/archive/2009/08/27/3277616.aspx
Hope this helps.