Try the app "Disk Utility" - it is a Red Hat product included with many distros - I have it on Ubuntu 10.04. It shows one of my drives as 15K (correct) and another at 7.2K (correct) and the other as - (incorrect). They are all Western Digital drives of about the same vintage - not sure why one does not show the rotational speed. But 2 out of 3 ain't bad
The utility will also show the model #, serial number and a whole lot of other info. You should be able to search on the model number and find what the speed is.