How Can People Tell What is AI?

Using Chat GPT, I created (or IT created) this poster for an upcoming workshop I’m doing at our local library.
I sent this to the librarian to get her feedback, and she said that they had to have their in-house marketing person create the poster with their logo, etc. That makes sense. But then she said they had recently shared a poster online about a library foundation event and had received “lots of negative comments” because it was AI.
I didn’t ask her how she knew an AI made this, but can you experienced folks tell me how she knew? I’m not trying to sneak anything by. I was just trying to make a nice marketing poster.
Nevermind about the Butlerian Jihad that slammed the library for using something created with AI. I just want to know how the librarian could tell I hadn’t “created it myself”, whatever that means.
Thanks!