How would I find out it my Mac mini has a virus or malware?

You can download the free malware removal utility here > https://www.malwarebytes.org/

Or, if you would rather not download MalwareBytes, follow the instructions removing malware manually here > Remove unwanted adware that displays pop-up ads and graphics on your Mac - Apple Support

First, never use any kind of "anti-virus" or "anti-malware" software on a Mac. That's how you cause problems, not how you solve them.

It's best to describe the functional problem, if you have one, in as much relevant detail as possible, rather than to say what you think is causing it or how you think it should be solved.

If a popup on a web page, or a telephone caller, told you that you had a "virus" or that anything else was wrong with your computer, it was a scam. There is no "virus."

It is unlikely you would have a virus. Malware is most likely adware pop-ups in your browser. OS X has built-in malware and virus protection that is updated when you update OS X. So you should keep your computer current with OS X. For the adware problem see:

Fixes for Adware and Pop-ups:

1. Malwarebytes
2. DetectX 2.11
3. Remove adware that displays pop-up ads and graphics on your Mac
4. Stop pop-up ads and adware in Safari - Apple Support