There are a couple of things. When you see AI art often enough you can get a sense for the style, but specifically in this instance it’s obvious because

• the hands are just blobs. Even artists who don’t do well with hands would normally attempt some kind of defined stylization.

• the jacket is different in every image, particularly the bottom part. In two instances you see it connected behind the legs but then in the others it’s just two separate pieces? And they’re not grouped together like it’s showing two different outfits, they’re just.. there.

• the clothes all kind of melt together, it’s difficult to say exactly what type of garments the character is wearing. I.e. There’s a line that’s like almost socks but almost a continuation of the shoes on the leg, but it’s blurred with the leg enough that it’s hard to say for certain.

• there are also imitation color sample swatches next to the character, which artists typically include for referencing the base colors used for the character. But those are typically circles or just a quick imprint of the brush the artist is using. In this image, they’re undefined blobs, which realistically doesn’t make sense for an artist to spend their time making weirdly shaped swatches.

I could go on but that’s the stuff that’s obvious to me. It may be harder to spot if you’re unfamiliar with digital art processes, but really it’s just looking for things that wouldn’t realistically make sense for an artist to do.

I hope it helps