They can't unless artist states so, like in this case where twitter profile makes mention about it. The rest now is a guessing game. Overall too perfect? AI. Bad anatomy? AI. Natural wardrobe skin wrinkles over a cosplay photo? AI. Do I hate this artist and my family tree roots back from Salem? AI. There are also some guidelines on the forum, but the line is too thin it will not matter anymore later.
Aside from the artist actually admitting it, the obvious tells in this example are the three shoulder straps, which is a mistake no human would make, and the obligatory melty mess that is the hair. You can see in the bottom left how the hair transitions into bedsheet wrinkles, which, again, is not a mistake a human makes. Other signs in general are muted color palletes and wonky backgrounds.
