You might observe the LED indicators on the system to get a visual indication about the status of the slot's hot-plug operation. The LED behavior, in case of PCI Express, matches that defined in the PCI Express specification or the behavior might otherwise be platform dependent.
Please refer to your platform guide for specific details. In case of PCI Express, when the Attention Button is pressed, the power indicator blinks, which indicates the beginning of a state transition. The blinking ends when the state transition has ended.