They usually do a noticeably bad job at kerning and text in general. They’ve gotten much better but it still doesn’t quite match the results of actually using a font/computer-generated text.
They also tend to not lay things out in quite the pixel perfect way you’d expect from a good designer. But that’s not as good a tell because many designers also fuck this up.
I probably wouldn’t catch these good examples as AI in a casual context, but those tells would be enough to decide if I was asked.
To me, it seems more to be something about the colors, but other than saying that they are often oversaturated, I can't quite place exactly what it is. I sometimes try to open them with an image editor and zoom in, or use other tools to try to "discover" what it is that I'm seeing, but without success so far. It doesn't help that graphics is not something I have experience with.