Reddit reviews The Radio Amateur's Handbook

The chances of getting a fake unintentionally are quite low, if you directly from Amazon and check that it says under brand "Microsoft", but id also check the reviews.

There is a good guide on eBay on how to spot the more convincing fakes.

Here are some fakes if you are looking to intentionally buy one: 1 2 3

Amazon US manage to keep sellers from using misleading language in the title, and usually require the seller to put a brand name other than Microsoft at the beginning of the title, unfortunately on the UK site it is more common, as you can see here.

Many modified controllers use fake parts, because the fakes usually have test points that can be used, making it much easier. Often the seller will use a genuine case and a fake circuit board, meaning there is no way to be 100% certain without opening the controller.

Fakes should always feel lighter than a genuine controller, however I once purchased one, where the seller had ripped out the rumble motors and put a ziplock bag of sand in each handle.