How do I know which GPS my Android Auto is using Apparently Android Auto can either use the GPS in the connected head unit or the GPS in the mobile device itself. As I understand, Android Auto will always prefer to use the GPS in the connected head unit. (which makes sense: typically has better antenna reception, doesn't drain the mobile unit's battery, etc).

How can I tell which one is being used ?

When the phone is in the car, I put it in "The Grotto" below the climate control and connect it via USB. I call it "The Grotto" because it is really deep and I'm thinking there's no way a GPS can reach in there. Yet my Android Auto works fine and never loses GPS signal. So something else must be going on.
According to this forum post it uses the phone's GPS. Can you see if the GPS indicator shows up on the notification bar? Also if it were the car's GPS, revoking location access to the app shouldn't cripple it. You could try this if you wanted to truly confirm.

(I have not used Android Auto before)