Ok, maybe I should have said that controllers often have no way to accurately determine the groundspeed of an aircraft from the radar.
The target trail can give an indication of speed but can also be misleading (and if you're relying on radar data only, even the g/s readout can be misleading with some manoeuvres). Depending on the type of processing and display the controller can sometimes also make an educated guess at the type of aircraft from the 'strength' of the return.