The most fail-safe method of determining actual network communication status is to use a packet inspector on a second computer connected to the same network. Using a tool such as Wireshark you can filter all communications for the IP address of the suspect machine, and then view, in real time, or by log analysis, what sort of communication is occuring at any given time.
Because this setup involves a second machine, it would be nearly impossible to conceal the actual network communication of the suspect machine.
A bigger question you ought to ask, though, is why would anybody hack your computer?
If indeed there is a malicious tool on your computer communicating over the network interface and using various methods to conceal that communication, it is much more likely you've simply got some sort of spam bot or worm software that was installed, not by hackers, but through more pedestrian and common forms of malware infection: opening the wrong file, downloading the wrong file, visiting a site on an infected server, etc.
Are you experiencing specific issues that lead you to believe that what the indicator icons and lights are telling you something incorrect? – music2myear – 2011-11-09T19:37:16.140
@music2myear I've told people to rely on that , but I realized that it perhaps it's not reliable at all. – Hernán Eche – 2011-11-09T19:41:41.920
1The odds are highly UNlikely that the icons and indicators are incorrect. Most viruses that do high levels of network communication do not attempt to hide the communication at this level, assuming, correctly, that most people don't look at their network indicator lights. – music2myear – 2011-11-09T19:45:01.090
Ya, to modify the icon, you'll not only need admin credentials, but you'll also need to bypass the Windows Resource Protection services. But if you can do that, you've got much larger options available to you. – surfasb – 2011-11-10T04:36:35.157