how to tell official taxi for great wall visit instead of unofficial taxi?

It's very difficult for you to tell the official and unofficials. Every official taxi has passenger service license. And there is a QR code on it. If you scan the QR code, you can learn about the basic information of the driver and the car. If you can't get the information, then the car is unofficial. But I am afraid that it's difficult for you to tell if you can't read Chinese.