Same way I can tell that my next door neighbor isn't poisoning my water supply or that Tesco doesn't have a secret chemical weapons program. That major supermarket chains are running scam charity appeals just isn't a hypothesis worth entertaining in the absence of any evidence for it.

Because the risk of fraud is too great for major corporations. They have accountants and businesses people who would report it.

The risk with charity money is not the companies collecting a few cents but the charity itself.