My understanding is that they present people with two images, the real original high-res image, and the upscaled image, and ask "Which of these is the real imag

My understanding is that they present people with two images, the real original high-res image, and the upscaled image, and ask "Which of these is the real image". The extent to which people are confused demonstrates how good the algorithm is. If it was perfect, and there was no difference between the original and upscaled then you'd expect people to pick each image about 50% of the time randomly.