How do i know if my call is going through WIFI or Cellular

Have you enabled WiFi calls? Has your Carrier made the feature available on your SIM? Have you contacted your carrier to know if the WiFi that you have has compatible with your cellular service provider's WiFi Calls?

If the answer is yes to all the above three questions then all the calls will be made through WiFi only if WiFi calling is enabled on your device and the WiFi Router is connected to the internet.