Google Voice Indicator
As far as your phone is concerned a Google Voice call is no different than any other call, so your only options involve Google Voice settings:


Set Make and receive calls = Prefer Wi-Fi and mobile data. This will make your phone treat the calls as VoIP calls which explicitly say the service they're using. This is how I use Google Voice and the quality is typically better than using the carrier service, which I have also used extensively in the past. Also iOS 12 improved the way VoIP calls are handled in the UI to make them work just like regular calls when your screen is locked (same phone UI as a carrier call).
If you want to use your carrier service,  you can change the incoming caller ID to show your Google voice number instead of the caller's number. This way you always know it's a Google Voice call, but you won't necessarily know who it's from. This is also why I prefer option 1.