(IDG) -- The average corporate network is attacked by hackers 12 to 15 times each year, according to a survey by the Computer Security Institute and the FBI.

Of the 563 users polled, 73% said hackers had penetrated their networks. But 18% said they had no idea if, or how often, their systems had been invaded.

In response, many security consulting firms are training their clients to recognize the tell-tale signs of a system or network invasion.

In some companies, up to 98% of attacks go undetect-ed, according to the Defense Information Systems Agency (DISA) in Washington. Even in security-conscious government agencies, more than 70% of the test hacks DISA conducted went undetected.

Attacks such as E-mail bombs and viruses are obvious, but few companies can detect a logical attack -- a planned invasion of the network, according to Winn Schwartau, a partner at The Security Experts, Inc. in Seminole, Fla. The consultancy attacks the networks of its clients to find security holes. "In the last six years, we've performed about 2,300 sanctioned hacks for our clients, and we've only failed to penetrate the networks twice," Schwartau said.

Gary Loveland, a partner at Price Waterhouse LLP's Information Security Risk Management Group in Los Angeles, said there are several obvious things to look for. They include unknown accounts added to the system and file server, an excessive number of log-on failures and dial-in attempts, any unexpected system or network crashes, unauthorized changes to system software and system files or high system activity when no users are logged on, especially during off-peak usage hours.

"Once they've successfully penetrated your system, hackers frequently create accounts for themselves so they can continue to get back in. And they typically will also attempt to give themselves administrator-level backdoor access into the network," Loveland said.

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Bob McKee, director of information management security at The Hartford Insurance Co. in Hartford, Conn., said his firm has a "very formal" set of policies and procedures and makes those policies a part of every new employee's orientation. On the product side, The Hartford proactively monitors and tracks all network activity via a series of firewalls and intrusion detection and auditing packages that can detect "attacks in progress."

McKee's group of 22 security managers also performs regularly scheduled audit and inventory checks and maintains strict password controls.

For further protection, the company has established a demilitarized zone for its World Wide Web servers to effectively isolate the corporate networks from the Internet. "There is no such thing as being too alert or vigilant," McKee said.

"If you don't have at least a basic detection mechanism, [for example] audit trails, intrusion detection, behavioral anomaly detection, you'll never know if your systems are hit," Schwartau said.