Even when you know what you are looking for, you are basically pixel hunting for artifacts or other signs that show it’s AI without the image actually looking fake, e.g. the avocado one was easy to tell, as ever since DALLE1 avocado related things have been used as test images, the https://thispersondoesnotexist.com/ one was obvious due to how it was framed and some of the landscapes had that noise-vegetation-look that AI images tend to have. But none of the images look fake just by themselves, if you didn’t specifically look for AI artifacts, it would be impossible to tell the difference or even notice that there is anything wrong with the image to begin with.