This is oddly useful information when you start to run into a situation where you’d like to scale a SharePoint server, but find nuances that it may be a standalone installation.
Look in the registry under:
"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shared Tools\Web Server Extensions\14.0\WSS"
There will be an entry for "ServerRole" which will tell you the installation type
Stand Alone – SINGLESERVER
Farm Stand Alone – WFE
Farm Complete – APPLICATION
Thanks to this forums post: http://social.msdn.microsoft.com/Forums/en-US/sharepoint2010general/thread/a498bb63-c706-471a-ad3f-1e6c63d4e3cc/