Open Management Studio and run the following query:
SELECT SERVERPROPERTY('productversion'), SERVERPROPERTY ('productlevel'), SERVERPROPERTY ('edition')
If SQL 2008 R2 is installed, the following results should be returned:
10.50.1600.1 RTM Enterprise Edition (64-bit)
The following would indicate SQL Server 2008, with SP1;
10.0.2740.0 SP1 Enterprise Edition (64-bit)