Malware on my Mac?

How can I tell if there is malware on my Mac? I get a pop up saying to call 877-****-9321 for support but this number is not listed for Apple Support. Thanks in advance.

Avoid phishing emails, fake 'virus' alerts, phony ... - Apple Support

If you suspect you have installed adware/malware. Try running this trusted utility https://www.malwarebytes.com/mac/

I wouldn’t say from a hacker, more like a tech support scammer. Could also be possible you have adware. But the thing is are you seeing these when you’re browsing the web or when you’re using an application like Word or Keynote or App Store? Because if you’re getting those messages when in an app like those three, you have adware. But if you seeing it when browsing the web, it’s just a webpage trying to make you call someone who’s going to say they’re tech support or something.

It is a spam call from a hacker.

It's a well known scam.