How to detect a hacker attack

Most computer vulnerabilities can be exploited in a variety of ways. Hacker attacks may use a single specific exploit, several exploits at the same time, a misconfiguration in one of the system components or even a backdoor from an earlier attack.Due to this, detecting hacker attacks is not an easy task, especially for an inexperienced user. This article gives a few basic guidelines to help you figure out either if your machine is under attack or if the security of your system has been compromised. Keep in mind just like with viruses, there is no 100% guarantee you will detect a hacker attack this way. However, there's a good chance that if your system has been hacked, it will display one or more of the following behaviours.
For windows:
1. Suspiciously high outgoing network traffic. If you are on a dial-up account or using ADSL and notice an unusually high volume of outgoing network (traffic especially when you computer is idle or not necessarily uploading data), then it is possible that your computer has been compromised. Your computer may be being used either to send spam or by a network worm which is replicating and sending copies of itself. For cable connections, this is less relevant - it is quite common to have the same amount of outgoing traffic as incoming traffic even if you are doing nothing more than browsing sites or downloading data from the Internet.
2. Your resident antivirus suddenly starts reporting that backdoors or trojans have been detected, even if you have not done anything out of the ordinary. Although hacker attacks can be complex and innovative, many rely on known trojans or backdoors to gain full access to a compromised system. If the resident component of your antivirus is detecting and reporting such malware, this may be an indication that your system can be accessed from outside.
Unix machines:
1. Suspiciously named files in the /tmp folder. Many exploits in the Unix world rely on creating temporary files in the /tmp standard folder which are not always deleted after the system hack. The same is true for some worms known to infect Unix systems; they recompile themselves in the /tmp folder and use it as 'home'.
2. Modified /etc/passwd, /etc/shadow, or other system files in the /etc folder. Sometimes hacker attacks may add a new user in /etc/passwd which can be remotely logged in a later date. Look for any suspicious usernames in the password file and monitor all additions, especially on a multi-user system.
Regards!