I am sure there is no way by which a script can guess if a webpage is not the original one.
It can work if the webpage is marked by someone using the extension as fake.
It'll work like a webpage abc.com looking like Facebook. A user ends up on abc.com and realises that the page is fake, he'll click the button and the url will be blacklisted straight away (or after someone's manual approval)
So, when another extension user will end up at abc.com, as abc.com is already in our blacklisted database, I can show the message however we want to do.
Let me know if I'm getting it right or wrong..
You got a 5.19% upvote from @adriatik courtesy of @jmehta!
You are mostly right.
Google has an anti-phishing extension that does not try to detect fake Google sites. Instead it detects when a user use their Google credential on a non recognise Google website.
Sometimes, you could analyse the source of the page and see if the current page contains a recognised pattern and if the domain name is not whitelisted then show a warning.
All those methods are not 100% accurate or effective which is why I use a combo of methods to try catch as much cases as possible.