it's ironic, since AI generated always looks polished - but the identification is mostly context-based i.e. we know nobody would pay anyone for making that illustration from scratch: because it's a throw-away

illustrations will be ubiquitous but mostly shit, only the shit will be more polished

so if an illustration is highly polished but otherwise garbage, it's AI with high probability - because the craftsmanship of the generator exceeds the artistic taste and development of the user