you cannot scan from the FortiGate. If the FortiGate is directly connected to the network, you would be able to check the arp table to see which client was connected in the past 5 minutes or likely still is.
Also, just to add what Markus suggested, you can also use device detection on FortiGate which will actively and passively scan for the network devices. But, this will only be possible for directly connected networks and not for remote networks.
