Some power supplies provide a "Power Good" signal when they are turned on and send out a " Power Fail" signal when they are turned off. This is usually used for monitoring and controlling purposes.
Power Good: after the output of a power supply reaches 90% rated voltage, a TTL signal (about 5V) will be sent out within the next 10-500ms.
Power Fail: before the output of a power supply is less than 90% rated voltage, the power-good signal will be turned off at least 1ms in advance.