It is not difficult to diagnose cruise control problems. A car's cruise system is composed of four separate parts, involving both mechanical and electrical systems. Should your cruise control fail, you should do some troubleshooting before you take the car into an auto repair shop. Most problems with the cruise control can be fixed very simply.
The vacuum system is the most common component to fail. To diagnose problems with the vacuum system, you will need a vacuum pump. Use the pump to apply vacuum pressure. It should release when you press the brake pedal.
Wiring - To test the wiring, you will need to be able to access the cruise control connector. This is located behind the dash, and will require that you remove a trim panel to access it. You want to use a multimeter and test each of the eight connections in the connector.
The steering column switch is another component that rarely fails. If everything else is correct, you will want to check this switch. Hook up your multimeter and ensure that you get a reading when the switch is depressed. If you do not, then you likely have an issue with this switch.
Hope this helps