There's nothing that I have found that indicates that a component is a master component or not, other than the green outline(instead of blue) only when selected, and right-clicking every component and seeing whether "Edit Master Component" exists or not in the context-menu. These two current indicators are not helpful at a high level because you have to select or click on each individual component instead of being able to see this information at a glance in the component/layers list.

The layer/component icon changes when the group is open(revealing the layers inside) or closed. This is not a necessary change because we can clearly see if the group is open by seeing the layers inside listed or not. Why not have the change in icon symbolize something useful like a master component vs. an instance instead?