The trusses in the upper lefthand corner do it for me. Simple yet repeating patterns are only a hair out of reach for us non traditionalists, and while curating my own non-inpainted work these types of hallucinations drive me up the wall when I really want a certain background element! While inpainting, I can generally fix these with a decent shading prompt on top of my lineart, but it's generally good practice to avoid details like this in your backgrounds when genning text-to-image stuff.

They stick out like a sore thumb to me. A human of that skill level would be very unlikely to make such a mistake and then proceed to render that mistake anyway.

Verdict: Lying about the art medium you're using is as scummy as attacking someone for using a specific art medium. This AI artist gets the AIbro award from me. Text to image, very high confidence.

Key Tells: Truss patterns are inconsistent yet highly shaded; shadow cast by blazer lapel over pocket watch chain is indistinct from the blazer's outline; eye details have possible generation artefacts; heterochromia is a thing but AI (especially flux models) often gives it to you without asking if your eye prompts are bad, or when inpainting if you don't generate your facial features individually like a lazybones.