How to know source of HR reading (strap or wrist) if I miss the external HR connection notification?

Look at the green light of the watch, if it's blinking your hr is being read by the watch, if not it's from the strap.

Marvin said:

Look at the green light of the watch, if it's blinking your hr is being read by the watch, if not it's from the strap.

Just to clarify, the green LED's should be solidly on during an activity if the reading is coming from the watch. If they are strobing, it probably indicates that the reading is coming from the strap, but the LED's remain on as a security measure if you have recently used the Garmin Pay function. If they are off, then it is definitely coming from the strap.

You can always check if the sensor is connected by navigating to "Sensors & Accessories". 

SInce I always wear the strap when doing activities I set the activities power mode to "Jacket Mode". That way the wrist sensor isn't used and I can be sure that if I see the heart rate it's coming from the strap and not the unreliable wrist sensor.

By green LED you mean the actual wrist sensor, not anything on the screen, right? I find that a bit hacky...

I realize that after selecting an activity and before starting the activity, when the options menu is still on the top of the screen, if the strap sensor is connected and reading, a heart icon is displayed there.

But after starting an activity I can't find anyway of accessing that information again, through the screen.

Hold "Up", navigate to "Sensors& Accessories" and check the status of your external HRM. It should say "connected". A bit clunky, but it's the surest way to know.

I don't know which external sensor you're using, but I can say that I've never had a disconnect (yet) when training and I almost never use my wrist HRM (only for walks and hiking).

That's a very good idea. Thanks.