Hello Alan and welcome to Seven Forums.
If I'm understanding your question, you want to know how you can tell if your computer is infected when you don't have any anti-virus program installed. I went to a couple of respected anti-virus company websites to see what they have to say: Symptoms | Infected Computer Computer Virus Symptoms How To Detect Malware Infection On Your Computer | Lavasoft
And a couple of general information articles: HowStuffWorks "How to Know if Your Computer is Infected with a Virus" How can I tell if my computer has a virus? How to tell if your computer is infected with spyware How to prevent and remove viruses and other malware
Personally, if you're on the internet, there's no good reason to operate a computer without an anti-virus suite. Even if you're not
connected to the internet you should still have an anti-virus suite installed. Maybe someone shares a flash drive with you and it has music or videos on it. Maybe it also contains a virus or other malware. There are so many free products to choose from. Here's a Seven Forums' thread: What's the Best Anti-virus?
Or you can google best free antivirus products
and you will find all kinds of reviews. Here's just one: Best Free Antivirus Software
If I misunderstood your question, I sincerely apologize. If you give a bit more info about what it is you're trying to find out...