There are plenty of tools that can show you which programs generate network traffic and through which ports they do so. That could be enough to find unauthorized traffic. However, if you really want to know the bottom line, you analyze the traffic yourself. You can with Wireshark. Wireshark is a so-called network protocol analyzer; a program that examines in detail what is happening in your wired and wireless network. That is of course quite a lot and so you can filter the information extensively. Wireshark can even analyze VoIP and GSM traffic. For advanced users only!
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WireShark – The Ultimate Network Traffic Analysis Tool
