Thought I'd blog this as an Aide mémoire more than anything.
Determine which Site Systems are present in the Hierarchy
select distinct ServerName as SiteSystems from v_SystemResourceList where servername not in (select servername from v_site)
Notice that I'm referencing views and not tables here which is good practice when querying an SMS\ConfigMgr database.
Read the complete post at http://wmug.co.uk/blogs/r0b/archive/2011/03/31/sql-query-for-site-servers-and-site-systems.aspx