To find what version of Exchange you are running, open ESM and click the server folder and the right pane will display the server build number, below is a list of build number and what Exchange version they are:

Exchange Server 4.0 4.0.837
April 1996
Microsoft Exchange Server 4.0 (a) 4.0.993
August 1996
Microsoft Exchange Server 4.0 SP1 4.0.838
May 1996
Microsoft Exchange Server 4.0 SP2 4.0.993
August 1996
Microsoft Exchange Server 4.0 SP3 4.0.994
November 1996
Microsoft Exchange Server 4.0 SP4 4.0.995
April 1997
Microsoft Exchange Server 4.0 SP5 4.0.996
May 1998
Microsoft Exchange Server 5.0 5.0.1457
March 1997
Microsoft Exchange Server 5.0 SP1 5.0.1458
June 1997
Microsoft Exchange Server 5.0 SP2 5.0.1460
February 1998
Microsoft Exchange Server 5.5 5.5.1960
November 1997
Microsoft Exchange Server 5.5 SP1 5.5.2232
July 1998
Microsoft Exchange Server 5.5 SP2 5.5.2448
December 1998
Microsoft Exchange Server 5.5 SP3 5.5.2650
September 1999
Microsoft Exchange Server 5.5 SP4 5.5.2653
November 2000
Microsoft Exchange 2000 Server 6.0.4417
October 2000
Microsoft Exchange 2000 Server (a) 6.0.4417
January 2001
Microsoft Exchange 2000 Server SP1 6.0.4712
July 2001
Microsoft Exchange 2000 Server SP2 6.0.5762
December 2001
Microsoft Exchange 2000 Server SP3 6.0.6249
August 2002
Microsoft Exchange 2000 Server post-SP3 6.0.6487
September 2003
Microsoft Exchange 2000 Server post-SP3 6.0.6556
April 2004
Microsoft Exchange 2000 Server post-SP3 6.0.6603
August 2004
Microsoft Exchange 2000 Server post-SP3 6.0.6620.5
March 2008
Microsoft Exchange 2000 Server post-SP3 6.0.6620.7
August 2008
Microsoft Exchange Server 2003 6.5.6944
October 2003
Microsoft Exchange Server 2003 SP1 6.5.7226
May 2004
Microsoft Exchange Server 2003 SP2 6.5.7638
October 2005
Microsoft Exchange Server 2003 post-SP2 6.5.7653.33
March 2008
Microsoft Exchange Server 2003 post-SP2 6.5.7654.4
August 2008
Microsoft Exchange Server 2007 8.0.685.24 or 8.0.685.25
December 2006
Microsoft Exchange Server 2007 SP1 8.1.0240.006
November 2007
Microsoft Exchange Server 2007 SP2 8.2.0176.002
August 2009
Microsoft Exchange Server 2007 SP3 8.3.0083.006
June 2010
Microsoft Exchange Server 2010 14.00.0639.021
October 2009
Microsoft Exchange Server 2010 SP1 14.01.0218.015
August 2010

ref:http://support.microsoft.com/kb/158530