I was suprised that I couldn't find a complete list of "build number to service pack" mappings for SQL Server 2000 Reporting Services anywhere... So, here they are:

2000:

RTM: 8.00.743.00

SP1:  8.00.878.00

SP2: 8.00.1038.00

SP2 with KB901383 Hotfix:  8.00.1042.00

2005:

RTM: 9.00.1399.00

SP1 CTP: 9.00.1528.00

To check your version just surf to http://yourServerName/reportServer ....then scroll to the bottom of the page.