Heart rate chest strap & wrist during workouts, how to tell?

It will read from wrist if it loses connection but afterwards it will ask if you would like to download HR data from the strap. For this reason you can start an activity that does not need gps, leave the watch, and come back to it once finished.

Ok interesting, is there a way to tell its reading from the wrist? 

While active: Pull the watch up and check if the green light is on. - 

After the fact : I do not know how to tell if\when it switched back and forth but if you confirmed it was connected before the start (checking if the green light was off) and downloaded the HR after the fact if it asks, it is pretty safe to say it used the monitor

Edited

No, but the way to tell if HR was from the belt or watch's optical sensor in Garmin Connect...especially when you analyze after a few days/weeks/months.

Maybe as a suggestion to add some differentiation. Something like the model of the watch by the activity, same can be there Bluettoth HRM, or Garmin HRM RUN, DUO, etc.

Right yes a good tip to be checking for the blue light. 

Incidentally the readings are coming from the strap, on my last run i turned the strap off & got better readings from the wrist. I didn't expect the Garmin pro heart rate strap to malfunction so soon, its lasted about 160 workouts. Maybe its the cold weather, i did use gel that normally helps with getting a connection. I will try my more basic blue tooth strap & see how that performs.