Because there is no unique answer to your question, first because you didn't define AI art.
I can't identify for sure the raven image that was posted before as AI, but I had a hunch because it was trying to be photorealistic but had a very strong blur in the distance, which is a flaw of many models of a specific implementation of an image-generation AI. Nothing conclusive, just a hunch. But it was a method effective because it was a generation of a photograph-like image, made from scratch. It wouldn't have worked if there was a real photograph being improved/modified/touched up with AI. Would it be AI-art in your definition? The means to detect AI art depends (even if the case there are ones) depend on how the AI works, and therefore we need more definition to answer your question effectively.
Also, one does need to know the goal behind the question, because the answer will depend on it. If it's just to win "guess if it's real or AI generated" contests so prevalent these days, a wide test, with acceptable accuracy, will be a good answer. If it is something with higher stake, like sueing someone into oblivion, you'd need a 100% effective test that would modify the answer. Without this information, one need to rephrase your question to answer it. Or accept that the question can't be answered, which isn't leading to a discussion.
I proposed to ask, for each image, variants of the same character including some details that would be very difficult to have an AI engine trained on, like eating sushi upside down. It is not perfect of course, but it would be a hard test of consistency (a current weak point of many generative AIs) and it would be easier for a carbon-based artist to start back from a previous sketch. Inability to do that would make me tell something is AI art. Does it work for your use-case?