How do I tell if my iPhone is using wifi or not?

Look at the top of the phone, in the status bar, and if you see the wi-fi fan icon, then the device is connected to wi-fi.

You can see what it looks like in this page of the iPhone User Guide. http://help.apple.com/iphone/11/#/iphef7bb57dc

Turn off cellular data.

If you are able to connect to the Internet then you are using WiFi (unless you are tethered to a hotspot by Bluetooth or USB).

Also you will see the WiFi symbol at the top of the screen.

Go to Settings/Wi-Fi and it will display any network it is connected to. You can also go to the router's management page, which will list the connected devices and their status.

HOWEVER, when the phone is locked, Wi-Fi is turned off to save energy, unless the phone is connected to power. So you know that Wi-Fi is off when the phone is locked.

Look for the WiFi symbol in the upper left of the screen.

Check in the settings tab Wifi. There you can see a check mark next to the connected WiFi. Also, on the top left, a WiFi icon shows your connectivity and strength to the network.