Log onto server
Open SQL Server Management Studio (SSMS)
Click About then Help
To the right of SQL Server Management Studio match the Versions with the list below to determine your service pack level.

SQL Server 2008 Service Pack 3 10.00.5500.00
SQL Server 2008 Service Pack 2 10.00.4000.00
SQL Server 2008 Service Pack 1 10.00.2531.00
SQL Server 2008 RTM 10.00.1600.20
SQL Server 2008 R2 Service Pack 1 10.50.2500.0
SQL Server 2008 R2 RTM 10.50.1600.1