Can a reverse image search prove an image is not AI generated?

No. A reverse image search shows where else an image appears and roughly how early, which helps you trace a source. It cannot prove an image is not AI generated, because AI images are posted and copied like any other file, and an empty result only means the search found nothing. Treat it as one lead, not a verdict.

Ivy, your instinct is right. Google's help page describes what a search by image returns: results that can include websites with the image or a similar image, plus similar images. That is a list of where and what looks alike, not a statement about how the file was made. Source: Search with an image on Google, Google Search Help.

On the other side, the Content Authenticity Initiative says AI image detection tools can be valuable but their results can be unreliable, and that Content Credentials are not meant to prescriptively say whether content is "real". They record origin and history when they are present. So no single check settles it. Source: How it works, Content Authenticity Initiative.

For your log, write what you actually found: earliest appearance, who posted it, whether a named photographer or agency can be reached, whether Content Credentials are attached, and what you could not establish. Lou, a missing match is not a tell, as Remy said. For the wider checklist, see how to check if an image is AI generated.