In the beginning, there was the stoned virus that would provide a funny message "your computer is stoned." As you laughed and shared this with your friends, your files were being deleted. Then came the big Michelangelo scare of 1992. Files started to disappear and machines crashed all over the world. Big News...
So, how do I answer this the best I can? At some point viruses and Trojan's became smart by not showing themselves. This functionality provided the thrust for companies such as Symantec and McAfee to propagate their products around the world, leading us to our new position of protecting every layer of the network.
Now, how do you know when you have one of these nasty critters on your computer? There is no real direct answer except to have an antivirus, IDS, content or other system tell you a virus has arrived or has corrupted your system. Sometimes you will get a funny message "your computer is now nuked" or something silly, other times the antivirus product will find it... and finally, other times you simply stumble upon the darn thing when you are trouble shooting a problem.
I guess the very best answer would be due diligence to monitor your systems and provide the best possible solutions and maintain those systems. Knowing your systems and keeping them current is always the best way to determine if you have any problem.
For more information on this topic, visit these other SearchSecurity.com resources:
Virus Prevention Tip: Recognizing funny e-mails
Executive Security Briefing: War on viruses: An IT manager's perspective
Ask the Expert: Identifying malware on a system
This was first published in February 2002