Is there a way to check if the GPU is legit or not?
Why do you want to know?
If you are buying a gpu used, on ebay, you do get some protections if you have the ability to test it out.
Some fakes can be very sophisticated so who you buy from needs to have a good reputation.
Match up the specs with what you see on GPU-Z.
Search using the serial number to see if it matches.
There's tons of ways to spot fake products. What you should be really skeptical of is if it's a brand that you've never heard of. I have seen more than a few instances of this pop up on eBay and similar places. If a brand name sounds suspicious or you've never heard of it then you are right to be skeptical. There's a few known Chinese manufacturers that make legit products (such as Zotac) but there's also a few who you should avoid. This article does a good job of telling who is real or not:
How to spot fake graphics card. List of trusted or legit Chinese graphics card brands along with brands that should be avoided. Some of the reputable and top graphics card brands are Asus, EVGA, Gigabyte, MSI etc. Graphics cards from well known brands do cost a bit more than the less popular...