First of all, this is my personal opinion. At first you need to have access to the same network which the victim is using, it may be the same wifi public or privet or you can simply share your own wifi. There are so many apps for android or software for pcs available like Zanty. With those tool you can simply monitor the users of the network. Now you can the ip adddress or if you can't you can see the mac address. Now just send a request through mac address and victim'm machine will reply with the ip address. So Simple.