For me what always gives it away is not the posters themselves, it’s the context. These are all overdesigned for a local community fair. There is no way they’d be paying for a professional designer for this, therefore it’s AI.
On the other hand if it had word art, grammar/spelling mistakes, images with wrong aspect ratios or clipping issues, that would immediately convince me it wasn’t AI.
I think that's unfair. Many local community fairs happen to have a graphic designer or someone with some of the same chops who's willing to lend a hand to design a decent poster.
On the other hand if it had word art, grammar/spelling mistakes, images with wrong aspect ratios or clipping issues, that would immediately convince me it wasn’t AI.