If while browsing you come across a Gucci website, then the first thing which you have to do is to check the URL properly. See if the name of the brand in the URL is correctly written or is it incorrect.
The next thing you have to see is that is the website associated with the official Gucci Website. If it is not, then you have picked another proof of its fakeness.
Compare the website with the original Gucci website. This can be done very easily, go to the page where the out lets and the contact information has been written match it with the information mentioned on the official website. If the stated outlets and contact info is not matching, then another proof of bogusness has been revealed.
The last thing which you have to do is to see whether the links mentioned on the websites are working or not, because there are few links which seems original, but they do not work.
After spotting all phony aspects of the site you will have to visit the official website and send them an email about the fake site and confirm whether it is actually fake, or it is not.