How to identify that the file is a virus file?

Check this

Virus Behavior

http://www.cknow.com/cms/vtutor/virus-behavior.html

http://polymathprogrammer.com/2008/03/05/computer-virus-behaviour-thesis/

http://www.webroot.com/En_US/consumer/articles/computer-security-threats-computer-viruses

How to secure

https://www-secure.symantec.com/connect/forums/your-system-infected-symantec-tools-help-clear-infection

Best Practices for Troubleshooting Viruses on a Network

check this link

https://www-secure.symantec.com/connect/articles/using-symhelp-how-do-we-collect-loadpoint-logs-and-submit-same-symantec-technical-support-t

Check this artical

Eliminating viruses and security risks

Hi,

There is not any thumb rule for it. You can judge in some cases and if seen any real time activities on the System then can come to conclusion that it's infected. IT experience/System experience can help you to make the correct judgment.

Symantec does provide Symhelp tool to find out suspicious files present on the System.

Can refer this article: https://www-secure.symantec.com/connect/articles/using-symhelp-how-do-we-collect-loadpoint-logs-and-submit-same-symantec-technical-support-t

SymHelp tool is a good start

How to collect and submit to Symantec Security Response suspicious files found by the SymHelp utility