Is it actually possible to tell if your computer has been hacked? Sometimes it can be quite obvious, but when done by an expert hacker it can often be virtually impossible to determine if you do not have extensive ability in the field of computing yourself.
The first knowledge you may have is the sudden reduction of your financial accounts from a healthy balance to a nasty deficit. Or applying for something only to find someone else has already applied for it as you; what's referred to as identity theft. Not the most pleasant of surprises. Something I would presume we would all like to avoid.
Even relatively amateur hackers, if given the opening, can intrude into the inner workings of your computer. Computer, network and Internet security would not be the big business it is if this was not the case.
Protection through computer security methods and applications is not, however, the basis of this article. For your Internet security you need a firewall, a real-time anti-virus, a registry/system file monitor and at least one anti-spyware application as a minimum, particularly so if you use a Windows OS (operating system). If you do not have effective versions of these, you are lost before you start. Please do not consider it remiss of me to not go into further details on these in this article. For those readers desiring more detailed information, please see the articles under Helium's titles "The basics of home computer security", "How to protect your computer from hacking", "How companies can safeguard against hacking" or "How to protect a computer from malware".
The primary concern of this article is endeavoring to determine if your computer has been hacked, presumably in the hope that you can mitigate the effects thereof before they're too detrimental. There are telltale signs that possibly indicate certain types of hacking, but please be aware that it is best to defend from hacking pro-actively rather than try to compensate for the negative impacts of hacking re-actively. Hacking affects many people, almost always detrimentally, and therefore society as a whole. This can include international and national acts of terrorism.
Even strong indicators are not necessarily determinate. Reduced performance is often indicative of hacking, the hacker utilizing your computer's resources to process their own computations, thus reducing the capacity available for your own. There have been estimates that as many as a third of the personal computers in the USA have been hacked and are used in this manner by Organized Crime syndicates. Providing such organizations with an extensive and very powerful distributed network. There are, however, a lot of other possible causes that may reduce the performance of your computer from when you first purchased it.
The strongest indicator that a reduction in performance is due to hacking is its rapidity. Most "legitimate" performance degradation will occur slowly, particularly if related to a Windows operating system or hard-disk fragmentation. A recent infiltration by a hacker, who then utilizes your computer for his or her own purposes, will show as a rapid reduction in your computers performance.
But not always. If you leave your computer powered on and connected to the Internet at all or most times, the canny hacker will arrange their utilization to subordinate itself to yours. This means that you will NOT notice performance degradation because when you are using it, it will perform normally. When you are not using it, it works on behalf of the hacker. This is the predominant methodology used by Organized Crime within the USA.
Not working at all is another symptom that may indicate that your computer has been hacked, usually by the more malicious hacker or the juvenile "adventurer". This indicator is far more indicative if you use an operating system other than one of Microsoft's. Windows operating systems will on occasion freeze on you for no apparent reason. Unfortunately, this is NOT abnormal or necessarily an indicator of a hacking attack. Microsoft issues an appallingly high number of patches for their software, it would not be unreasonable to question whether they test their systems at all before marketing them.
The malicious hacker has extracted the information they want and do this to hinder your discovery of what they have done. The juvenile generally does it because they can, the lack of computer capability may be a hindrance for you, but data theft detrimental to your circumstances is unlikely. The problem is that you can't tell the motive of the culprit. Although, some juveniles will let you know, generally through annoying and pervasive messages that continually propagate.
This attitude from the juvenile hacker is fairly typical. From a technical point of view it may be relatively insignificant as often you can continue to use your computer after this sort of hack, but considered on the basis of annoyance it can be excruciating to the user.
As has already been mentioned, the one other "obvious" indicator that your computer has been hacked is when information that you considered confidential and only existing on your computer, no matter what it is, is used by someone unknown or has now become available to anyone who desires it on the Internet. Broadly dispensed confidential data from your computer is the most conclusive indicator of hacking you can possibly find.
As I hope this article makes clear, recovery from a hacking incident is problematical at best, at worst it is impossible and the hack may destroy you financially and professionally.
Do NOT wait.
Protect your computer systems, whether on an individual system or a LAN before they are hacked. There are many excellent software applications available to the Home user for free from reputable companies. Similar packages are available to small businesses for a relatively small charge that you should be able to write-off as business expenses. Medium to large companies should already be employing network security experts or at least hiring appropriate consultancy firms.