How to check if a memory card is real
With counterfeit cards, fraudsters program an SD card into thinking that its a 32Gb or 64Gb card when in actual fact, its usually an 4,8 or 16GB card. The normal behaviour of a counterfeit card is that when the genuine usable data has been used up, the rest of the files will appear as black, unreadable files up until the camera believes that your memory card is full. The way of testing this is to transfer all of your working data (300 photos) to a computer and seeing what the GB total is for that usable data. Next, transfer that footage to your computer, and see if you can take more videos or photos, and see if they re successful, or whether you start getting black files after the same amount of GB has been used up.
You can also perform a memory card check using a H2Testw. You just need to do a search online for the free tool. This test will tell you what memory card capacity your card has.
This forum explains the purpose of the H2Testw: https://www.avforums.com/threads/fake-sd-cards-test-yours-now.1347842/
You could also get in touch with the memory card manufacturer who will probably ask for the specifics of the card to ascertain its reliability.