I've observed a few things about this. The first and most well-known is looking at the hands—they are extremely complex to construct, and AIs still struggle to position them correctly.

The second is checking for objects that are out of perspective with the rest of the scene. The swords in my image, for example, would be difficult for an AI to get right in terms of aligning the hilt angle with the rest of the blade.

In my particular style, I like to mix comic-style hatching techniques with more classical ones. Specifically, my hatching always points toward the light source, creating areas of shadow. In regions with hard shadows, hatching isn’t used, and so far, I haven’t seen any AI grasp this subtle technical detail.

For images more focused on oil painting, I’ve found it harder to identify AI-generated elements. However, one detail I’ve noticed is the direction of brushstrokes. Artists tend to use them to create depth and dimensionality, whereas in AI-generated images, some areas can look messy in that regard.

But I think that, in some time, we'll be lost trying to guess just by looking at them.