How to confirm Heart Rate monitor source?

ThiagoDolabella said:

how can I verify that measurements are actually sourced from the strap rather than the watch?

Do you mean in real-time, or checking it later in Garmin Connect?

In real-time (in case you miss the message telling the strap was connected), you could simply remove the watch from the wrist, and see whether it is still showing the HR. If it does, then it looks like the strap is connected. You could also turn off the wrist HR sensor (for example with a hot key, or with a Power Mode assigned to the concerned activity profile). 

The connection of straps with Running Dynamics (like for example the HRM-Pro) can be detected and monitored easier, and even continuously with the help of an RD data field. For other straps there might be a Connect IQ app capable to show the presence of the strap in real-time (but I do not know any personally at this moment).

And if you mean later in GC, then some sensors may be listed directly on the activity screen (though I believe it works only with Garmin's HRMs). Otherwise, you can also look on the Devices screen » Sensors - the date and time of the last connection is shown there. There is also a possibility to find the information about sensors in the JSON data of the activity in Garmin Connect Web.