I had asked a similar question in a thread that I started yesterday. I wanted to know when my file was getting HR data from the wrist and when it was coming from the chest strap. This tool will be able to do that (although you need the paid version to see the second half of the activity): https://www.fitfilerepairtool.info
Note that you have to download your activity from the garmin website as a FIT. The source of the data line-by-line is not present in the tcx or gpx downloads.
There is a manual on there and some other instructions.
I was looking at this because I wanted to see what would happen if, during an ongoing activity, I took off my chest HR strap. Would my 935 be smart enough to switch to grabbing HR data from the wrist? I got very odd HR info after I did this, so I suspect it was still trying to read from the strap (which was in my bike bento box). When I get home tonight I'll be able to see exactly where each HR data point was coming from.
No, you have to open the activity in Garmin Connect Web, while looking for the data in the DevTools. But if you want to use the FIT file, it is also possible - install the FIT CSV Tool, and convert the FIT file to a CSV. You can also use some of the FIT tools available. For example Fit File Repair Tool, or others. Using the DevTools is the easiest I think, and does not require any software.
... have a look at this post, for the detailed instructions how to find it with the help of DevTools: