I even experimented with the pulse monitor on my treadmill.
it comes off the machine on the left so that's the side where I usually clip it onto my finger.
I tried it on all my fingers and then moved my arms to various positions of out and up. Just to see what would happen.

As soon as I got my arm at about 90 degrees out from my body the thing quit pulsing and I did it many times , to confirm - the same places every time.
The sides varied a little bit as my Left is worse than the Right side.