Without fully restating what’s already published, please read this MSDN Blog Post for more details on how to determine which SQL Client version is connecting to a SQL instance.
will tell you which version is connecting to your instance based on the TDS protocol appendix document, http://msdn.microsoft.com/en-us/library/dd339982(PROT.13).aspx.
SELECT CAST(protocol_version as BINARY(4))
SQL Server 2012 is almost complete, and we now know that the TDS protocol version for this will be 0×74000004.
Another option appears to be to use the client_version column in sys.dm_exec_sessions. A client_version = 4 is SQL Server 2000. 5 = SQL Server 2005. 6 = SQL Server 2008. 7 = SQL Server 2012.