What "genuine" means
It does not mean "expensive" and it does not mean "unique". It means the work is what the seller says it is: made by that person, in that technique, in that number of copies. A work costing a few tens of euros can be perfectly genuine; one costing thousands may not be.
The signature
Look at where and how it is signed.
- An art photograph is usually signed in pencil, on the front in the white margin or on the back. Pencil is a technical choice: it does not bleed through the paper and does not degrade the way many inks do.
- A signature printed with the image is not a signature: it is part of the reproduction. You can tell because it is at the same resolution as the image, with no relief and no variation in pressure.
- On a painted work, the signature is in the material of the work itself.
The certificate
A certificate of authenticity is useful if it carries verifiable information: title, year, technique, exact dimensions, edition number if any, and the signature of whoever issues it. A sheet saying only "original work" with a stamp adds nothing and proves nothing.
The strongest certificate is issued by the artist, or by the gallery representing the artist. One issued by a third party with no relationship to the author is worth exactly that third party's reputation.
