They know that because they took the seeds to the Mississippi State University to have them tested. The test came back revealing that the seeds the farmers had in their possession were not the same as the certified seeds that the company was selling. It's in the article but it seems that people might not have read that part. Whether or not these farmers were the only recipients of the fraudulent seeds is unknown. But so far there doesn't appear to be any white farmers bringing their own lawsuits on the matter or attached to the black farmers lawsuit as co-plaintiffs. It appears that only black farmers have had enough of a bad experience to warrant litigating the matter.