How can I determine the edition an Oracle session is using? We are using the Edition Based Redefinition feature in some software and as a DBA, I want to be able to determine which edition each given connection is using.  This way, we can be sure all connections to the database are using the correct edition.  How can I tell the edition being used by any given session?
Query V$SESSION and include the column SESSION_EDITION_ID which contains the value you are looking for.  

The link http://docs.oracle.com/cd/E11882_01/server.112/e40402/dynviews_3016.htm#REFRN30223 defines all the column meanings for that dynamic view; I would probably do a query like 

SELECT SID, USERNAME, SCHEMANAME, OSUSER, 
   PROCESS, MACHINE, TERMINAL, PROGRAM, 
   SESSION_EDITION_ID 
from V$SESSION  
order by SESSION_EDITION_ID;