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.
Prompt: make me a poster in the "looks like garbage made by someone who doesn't know how to use Microsoft word or what aspect ratio is" style...
Context is everything.
A twee handwritten sign for free range eggs at the roadside is likely to do well.
A handwritten sign for flying lessons on the other hand..