Suspicious? What's that?

ATLANTA, Georgia (CNN) -- Be the eyes and ears for the long arm of the law. Remain vigilant. Report suspicious activity. America is awash in Independence Day security advice.

There's plenty of instruction on what to do and even whom to call. The FBI has tip lines and the national homeland security center has added workers to monitor July Fourth events.

But is anybody watching? And what are they watching for? There's a deluge of instructions, but a drought when it comes to details. There's no clear definition of what constitutes something suspicious, but citizens need to be on the lookout for it.

Intuition -- even mother wit -- can be a guide, said Jeff Holmes, a spokesman for the FBI's Atlanta office.

EXTRA INFORMATION Gallery: What is suspicious?  Local Contacts  FBI Field Offices      QUICKVOTE Have you considered certain actions suspicious that you would not have thought so before September 11? Yes No View Results

"We want people to give us information but use a combination of common sense and good judgment," Holmes said.

CNN security analyst Kelly McCann defined "suspicious" as "visual unlikely circumstances that potentially add up to be considered for target value or as pre-incident indicator of an imminent attack."

Huh?

In plain speak: People making sketches or using video cameras inappropriately is suspicious, said McCann, a retired Marine major and specialist in antiterrorism.

Some other examples McCann offered are "people who are loitering in an area, evaluating the consistency or presence of security measures ... using note taking devices at visual events or a tape recorder at a visual event ... walking in a manner consistent with measuring or pacing."

But what do ordinary Americans think suspicious means? CNN.com asked several people what they thought.

"Suspicious means to me things that are out of the ordinary, that seem out of sync," said Maili Kessler of Fayetteville, Arkansas.

McCann would agree with that. "Part of the strength is the bad guy noticing that he's being noticed," he said.

"It reduces their confidence that they can do something and get away with it. Maybe avoiding the whole thing all together. It's a powerful thing."

But not everyone is watching. They've been told to look for something, but what that something is no one seems to know.

"Me, personally, I am not looking for anything," said Adam Brooking of San Jose, California.

"But I'm glad they do the alerts, so that people who are supposed to be watching, like policemen and security, can be alert. That makes me more comfortable." (Click here to read more comments)

Another question remains. Whom to call if there is something suspicious happening?

Some Americans believe they should call the universal emergency number. Others are not certain of whom to call.

"What comes to mind is 911, but I don't know if that is the most effective thing to call first," Brooking said.

McCann said 911 is an emergency system that should be called only if there is an "imminent attack unfolding in front of your eyes."

"The average person doesn't know that they can call the FBI regional office ... you can call any of the law enforcement offices out there to protect people," McCann said.

He also warned that Americans should not personally get involved if they think there is a potentially dangerous situation.

"You shouldn't intervene. No citizen should feel compelled unless they see something that is exactly what it appears to be," McCann said.

"A citizen should basically note first the descriptors -- height, weight, complexion, hair style, clothing, associated license plate. Then they should report it ... in a manner consistent with credibility."

In other words, McCann said, "In clear language say, 'This may be nothing, but let me tell you what I saw.'"