I ran exiftool on an image I just generated:

$ exiftool chatgpt_image.png

...

Actions Software Agent Name : GPT-4o

Actions Digital Source Type : http://cv.iptc.org/newscodes/digitalsourcetype/trainedAlgori...

Name : jumbf manifest

Alg : sha256

Hash : (Binary data 32 bytes, use -b option to extract)

Pad : (Binary data 8 bytes, use -b option to extract)

Claim Generator Info Name : ChatGPT

...

Exif isn't all that robust though.

I suppose I'm going to have to bite the bullet and actually train an AI detector that works roughly in real time.