how do i know if a warning is legitimate and not another virus in disguise?

It is a scam.

In the message which claims to have come from Apple they claim they discovered virus on your Mac. For starters neither Apple nor anyone else can tell if there are virus over it over the network. Apple would not offer to scan your Mac for you. They no better than that.

You can tell because no one can tell what's running on your computer remotely. Also, Apple does not monitor your computer. Apple does not send such scam emails.

Identify legitimate emails from the App Store or iTunes Store - Apple Support

Look closely at that web address. It is not even a valid Apple address that ends in apple.com. They incorporate apple.com in the real domain string that ends in .live, and a lengthy redirect string to their criminal intent server.

Immediate SCAM give-away that this is a very real looking, but entirely deceptive web page. The other comments are valid, about not being Apple, and no filesystem checking over the Internet. Or even any viruses to be found on Macs, for that matter.

You know it is a scam because it is trying to scare you into reacting.

You know it is a scam because nothing on the internet can even see your Mac, much less scan inside of it over the internet.

You know it is a scam because you thought, "hmm, I wonder if this is a scam."

You know it is a scam because the domain is trying to look like a valid domain, but it isn't. For it to be Apple, it must be apple.com, not apple.com-shield.live--notice the top level domain is .live, not .com.

It's a scam.