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Session:

dbus-send --session --type=method_call --print-reply --dest=org.freedesktop.DBus /org/freedesktop/DBus org.freedesktop.DBus.ListNames

System:

dbus-send --system --type=method_call --print-reply --dest=org.freedesktop.DBus /org/freedesktop/DBus org.freedesktop.DBus.ListNames

You can also use DFeet if you prefer a GUI tool.