This blend file contains a Sphere, a Cube and a Cone.
The action CubeAction acts on the cube. In the Action Editor, it can be seen to have 2 users. So it must also act on another object, but which one, the sphere or the cone?
In this case I can just just select the sphere & cone in turn to see which one causes the Action Editor to focus on CubeAction. But what if there were many objects?
Actions don't seem to appear as a node in the outliner that I could expand to somehow see what objects they are getting applied to.
Blender 2.83.3 on Linux