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I recently installed an EV charger and noticed that there is a low voltage data wire to the car in addition to the high voltage charging wires.
In addition to the duty cycle determining the max amperage that the car can draw, there is a simple signaling protocol between the EVSE and the car which controls when the EVSE can apply AC voltage to the plug.
Basically, the EVSE produces a 1 kHz square wave at +12V into an open circuit when the car is not connected. When the car is plugged in, the car has a 2.74 kΩ resistor to ground which drops the voltage to ~9 V. When the car is ready to charge, it adds a parallel 1.3 kΩ resistor which drops the voltage to ~6 V. In addition, the EVSE can signal an error state by asserting -12 V. This is discussed in (Wikipedia).
The duty cycle of the 1 kHz pilot signal determines the maximum current the car is allowed to draw:
This is discussed in the same Wikipedia section (and the above table is from there).
