For me, the backgrounds were interesting for their _more salient_ features rather than their ambience, e.g. unexpected smears of color, textures that vaguely looked like real things at a glance (like fabric or nature) but wouldn't stand up to scrutiny. They reminded me of the typical "mistakes" that you see when playing around with image generators.

My heuristic in picking the fake one was to examine the fabric - the fake ones are all wearing what seems to be Dan Flashes.