Detecting viruses after a fake lockup message on Macbook Air

This is not about detecting viruses (Apple products are not compatible with Microsoft Windows viruses and cannot run them).

This is about detecting scams.

Some key points in a scam:

1. The messages never mentioned you by name (they don’t know it).
2. Suspicious accusations (claiming you have a virus when they have no physical access to your device).
3. Threatens "account closure" or other urgent excuse in an attempt to get a reaction.
4. Never click an unsolicited link. This directs you to a scammer who is (most likely) working in an organized scam call center in another country.

That’s all the evidence you need to block, report and delete the message.

In closing:

If you receive a suspicious email or SMS text message that looks like it's supposed to be from Apple, please email it to reportphishing@apple.com. 

Learn how to ⇢ Recognize and avoid phishing messages, phony support calls, and other scams.

Good luck! 👋🏼😉

There are no viruses that affect macOS, so there is nothing to detect.

Fake "you have a virus" messages are scams as old as dirt: Phony "tech support" / "ransomware" popups and web pages - Apple Community

Also, any "security warning" that includes a phone number is likely to be a scam.

U. S. Federal Trade Commission – How To Spot, Avoid, and Report Tech Support Scams

"If you get this kind of pop-up window on your computer, don’t call the number. Real security warnings and messages will never ask you to call a phone number."