Simon:

To give you an idea, and I can't say how accurate it is, apart from looking at the mfr's specs for the keyboard (either by website or the underside), you may try checking Windows XP's device manager to get the ma rating.

Right-click on My Computer
Select Manage from the context menu
Click on Device Manager in the lefthand pane
Click on the + next to Universal Serial Bus controllers in the righthand pane
Find any of the USB Root Hub items and double-click it
Click on the Power tab at the top

Any USB peripheral connected to that port will be itemized in the lower box, indicating the name/type of device and its power requirement

I can't attest to the accuracy of XP's USB device power detection, but its a start.