Breathing muscles monitoring
Observing arm movement is a simple way to monitor which muscles are active when I breathe. To get a more precise indication, I cross my arms, with each of my hands gripping the side of my opposite shoulder, and let my elbows rest on the point where my upper ribs meet at the front of my body.
My elbows move forward when I activate my ribcage and chest muscles while breathing (as in 3-part breathing). But when I breathe diaphragmatically, my elbows remain static.
When I activate my shoulder and neck muscles (secondary breathing muscles) while breathing, my hands move up and down.