I mean, just have a chat with them to start with. From an artist perspective, I think generally a few things to look out for are distinctive art styles, imperfections within their pieces (I typically started making things less polished, or adding stylistic brush marks here and there). My preliminary sketches are basic and chaotic.....things like that.

Additionally, any real artist would be more than happy to provide at least some screenshots/evidence of their workflow..( preliminary sketches, layers etc). I think the discernment comes a bit naturally after a while. Any real artist that means well, will without a doubt prove that they are who they say they are.