Not foolproof, but a couple of easy ways to verify if images were AI generated:
- OpenAI uses the C2PA standard [0] to add provenance metadata to images, which you can check [1]
- Gemini uses SynthId [2] and adds a watermark to the image. The watermark can be removed, but SynthId cannot as it is part of the image. SynthId is used to watermark text as well, and code is open-source [3]
[0] https://help.openai.com/en/articles/8912793-c2pa-in-chatgpt-...
[1] https://verify.contentauthenticity.org/