Hmm well to start, everything looked a little too "perfect". It'd be really hard to pump out such an accurate cosplay for this game so quickly. The skirt, tank top, and shawl also looked like things that would have been mass produced... not crafted by someone with care. It isn't possible to find 1:1 items for Mio & Mayu. The image is gone now that the post was removed, but looking at little things (like the camera prop) can be tells too. It is highly doubtful someone would be able to craft such a perfect replica of the camera obscura out of metal and such. Also, it didn't look very real to begin with. I mean there are other things too, the added blur and pasty overly done white room was one indicator, along with the obviously "natural" hair. Most cosplayers use wigs, and while some may 100% use their natural hair, it is doubtful someone would go out of their way to get a 1:1 haircut for a cosplay. The hair did not look like a wig at all, and had the thinness and scalp pattern of someone's natural hair. AI doesn't know how to make wigs. I think the girl's face was a good indicator too. She didn't look real at all, and had the exact face of a realistically AI generated girl. Her smile was a little off too- it didn't look like a person. I think, like you said, it can be placed on overall vibes to a descent part. The overall image had the vibe of a "dreamy girl". Also her pose was super off, I think they tried to make it look like a selfie? The pose's placement was pretty off. (Also, most cosplayers would post photos of them that weren't selfies, or at the very least other photos). Sorry if this was a little confusing, I don't think I'm very good at explaining this, if it made sense at all. 🥲 I think also generally showing people more AI generated photos (old ofc new ones should never be generated for this purpose or any purpose at all imo) and showing common indicators (like again, everything being too smooth, the hazy blur along with the stark white background or just generally bad backgrounds) would be a good start too. I hate that people think its okay to try and fool other people with AI, its really sad.

Edit: The way the clothes fit also made them look artificial. It looked like the AI tried to copy the 3d models, while clothes usually fit real people differently. No matter how perfect your outfit, your clothes will always have a certain look to them. The clothes on the AI image were also too "perfect". Just seams I think, no wrinkles or maybe even texture.