I want to detect all the android phone connected to my network by a wi-fi. I have used nmap but it is not showing any specific detail to get the android .can any thing is there by which I can detect at least a android is in my network.
Use MAC address vendor lookup and record all the MAC address prefixes for all the phone manufacturers that use Android. It's a heuristic but a pretty good one.
Only give a cert/key to devices that "register" with IT.
You shouldn't have unauthorized devices on your network.