Tool/software:
The receiver detection is part of the transmitter and it must correctly detect if there is a DC load impedance - 40 to 60-ohm or lower - is attached at the output of the transmitter.
A transmitter - while at high impedance - changes the common mode voltage on plus and negative lead to pulse a step voltage. Based on measuring the voltage rise time the device determines if a receiver is attached.
Once it detects there is a termination on its output, then it turns on termination on its RX pins - 40 to 60-ohm.
Please refer to PCI Express 2.0 Base specification - for further details related to voltage step change and other details:
Regards, Nasser