Arduino should check the same way like a USB keyboard or a USB mouse, if turned on notebook is connected I have a USB HUB and have connected to it a USB keyboard and a USB mouse. As long I have not connected a Notebook to it those devices don't turn on. 5V is always turned on! So I could charge my phone. How do they know there is no notebook connected? My Arduino is always on. I want to emulate this with an Arduino. Without the need of extra software on a notebook, the Arduino should be able to tell if a notebook is connected to the USB Hub or not. Please help.
With Arduino Nano, you don't have control over the USB, because it is handled by the USB-to-TTL-Serial chip on the board. The MCU is to this adapter connected only with RX and TX of the Serial interface. 

With Arduino Micro you could retrieve the state of the USB connection, because he MCU ATmega32u4 has a build in USB support an handles the USB connection from application (sketch).