I have been flying phantoms for 4 years.
I started with the phantom 2+. I have never seen a Fake Phantom. I have only seen other companies try to resemble their design to a phantom
There are however many. CHEAP CLONE drones of the MAVIC.
Keep price in mind. Of the price is too good to be true do a little more research before you buy.
Aside from the additional sensors, the vents on the underside look completely different. Do a Google search 4 complete photographic differences.
Currently on eBay there is a phantom 4 copy that comes in colors for around $50 that is quite popular and many have been sold. pretty sure those people are extremely disappointed when they try and fly it!
I would probably say it is easy to spot, the really cheap clones don't have the meaty large motors that the phantom uses and instead have those small long cylinder motors with a gear that spin another gear that drives the propeller. I would be careful on eBay or craigslist type of sites, but if the seller is high rating/reputation I think it's safe to assume they are selling a genuine product.
More of a personal story but a good warning. As some people mentioned above. There are some people selling their Phantom 4s or 4 advanced but advertise it as the professional model. Unfortunately some people don't know the difference. A good way to spot that is by getting the seller to provide you with more images. Especially a close up of the camera lense. The FOV number is different between the models. 84 for the PRO and 94 for the regular 4. The sensors are a big giveaway too. I almost drove to buy a P4Pro from someone but I was able to spot it was the regular 4. After informing them of it, they have yet to update their listing. Probably gonna try to scam someone.
Good luck on your purchase, I owned the P4P+ and it was one of the best things I ever flew.