Look in the %epoc32%\data directory there is a buildinfo.txt file. Beta 3 which says ManufacturerSoftwareBuild Crystal COAK086.
The older SDKs will have a lower COAK number. 074 for Beta 2 and 066 for Beta 1.
RE: what's the latest version of the SDK?
I suggested to Forum Nokia that they should inform the current version number in their Tools and SDKs page. My request was heard and nowadays you can check the latest version number at Forum Nokia --> Tools and SDKs page.