Radar services comprise:
a) Separation of arriving, departing and en route traffic;
b) Radar vectoring;
c) Position information to assist in the navigation of aircraft;
d) Monitoring traffic to provide information to the non radar controller;
e) Assistance to aircraft crossing controlled airspace.
Before a controller provides any of the above radar services he shall either:
a) identify the aircraft, using either secondary or primary methods; or
b) have had the identity of the aircraft transferred from another controller.
On departure, the aircraft has to be identified, and the mode C verified - to make sure everything is correct. It is obvious that the aircraft calling is the aircraft on the screen, but still, the procedures dictate that the mode A must be validated, and then the mode C verified.
I'm sure people with a greater knowledge than me will also chip in 
DF