How do you become aware of network attacks you can't see?

Besides emailed alerts about viruses and the like, how do you security pros approach becoming aware of things that may be happening in the background with little indication.  Somehow, in most of these high profile attacks in which lots of data was carried, admins had no idea they were happening while they were happening.  How does one best react to that sort of threat.  What is the best way, as admin to not be in the dark about less obvious attacks?
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JohnBusiness Consultant (Owner)Commented:
Most of the attacks I have read about involve systems that had not been patched and secured. Most network devices now and Windows operating systems now have patches out to block security holes.

If hackers do get in, they must leave entry tracks in the log that are different from normal activity. A simple copy of a file to a remote computer or device leaves no tracks. I am not sure if Server Auditing can track simple copies.  If the hacker must access a system to create a copy, that will leave tracks as well.

Best approach, keep up to date on security bulletins (ZD Net is as good as not) and keep network, servers, and Windows patched.

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Check out the Critical Security Controls from SANS.

Some of the ones that might help identify an intrusion are:

Identifying unauthorized network traffic.
Identifying unauthorized software.
Identifying unauthorized devices.
Identifying unusual behaviors, or attempted security breaches via log files.
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