How to choose between wrist HR or chest strap HR?

Hi!
New with garmin products so sorry for dumb question.
I'm about to go run but how can I be sure that HR is coming from chest and not from wrist?
I've paired my heart rate sensor and also have wrist hr set to automatic. Do I have to disable the wrist hr when using chest strap or does the Fenix always read chest strap before wrist hr when training?