10.3.2 Determining the Lock State of the Security Register
The Lock state of the device can be determined by sending a subset of the Lock command to the device. Only the device address byte and word address byte need to be transmitted to the device to determine the Lock state. An ACK response to the word address byte indicates the Lock has not been set while a NACK response indicates the Lock has been set. If the Lock has already been set, it cannot be undone. The abbreviated Lock sequence is completed by the master sending a Stop condition to the device.