There’s no foolproof single test, but you can build strong evidence of authorship.

For photographers, the best proof is usually your original source material: keep the RAW file and the rest of the shoot sequence. A buyer may receive a JPEG, but they normally won’t have the RAW capture or adjacent frames from the same session. Embedding copyright/contact details in EXIF/IPTC metadata is also worth doing, even though metadata can be stripped. Publicly posting the image on an independent site that shows an upload date can help establish a timeline. For portraits, signed model releases can provide supporting evidence.

For buyers, verification is mostly about checking clues rather than proving ownership with certainty. Reverse image search is useful. Also look for consistent metadata, older public postings tied to the seller, and other images from the same shoot or portfolio that suggest they are the creator. Watermarks may help, but they can be removed or faked. Dates inside files are weak evidence because they can be changed; upload/publication history is generally more useful.

In short: creators should keep originals and documentation; buyers should look for a consistent chain of evidence, not rely on one signal alone.