There are different RAM brands of graphic cards on the market: Samsung, Micron and Hynix.
There is basically no issue with Samsung and Micron, but it’s another story with Hynix RAM. Indeed the Hynix RAM could make you lose up to 30% of hashrate compared to Samsung and Micron RAM.
Unfortunately there is no simple solution to this problem. Unless you are able to buy high volume of graphic card, no supplier will guaranteed the brand of the RAM in a graphic card. So it’s unfortunately a question of luck. Note that’s the Hynix cards represent 5% to 10% of the market.
There some solution to know the RAM of you graphic card:
On windows, you can use GPUz which is a benchmark software, allowing you to gather a lot of information on your graphic card
On Linux you can use GL Mark II which is a complete software which allow you to know the RAM brand of your graphic card