The Qt D-Bus Viewer is a tool that lets you introspect D-Bus objects and messages. You can choose between the system bus and the session bus. Click on any service on the list on the left side to see all the exported objects.
You can invoke methods by double-clicking on them. If a method takes one or more IN parameters, a property editor opens.
Right-click on a signal to connect to it. All emitted signals including their parameters are output in the message view on the lower side of the window.