To know who are connected to your wifi with easy and without login to the wifi/router device.
Use your smartphone Android or iOS and install an application named : Fing
Fing is a network scanner tool, easy in using, fast, and accurate, you will know the ip address and the MAC address (media access control address) of all devices connected to the Wifi.
Open Google Play Store (Android) or App Store (iOS), then search the app using keyword: fing,
then choose Fing-Network Tools from Domotz.ltd and install it.
After installed, run the Application.