Inspired by this statue of (ostensibly) Clementia, Roman goddess (figure from Wiki).
How do we know this is Clementia? The Wiki article on Clementia says "In traditional imagery, she is depicted holding a branch (possibly an olive tree branch) and a scepter and may be leaning on a column", but there is no branch nor scepter nor column in this statue.
In the same vein, how do we know who the subject of any sculpture is? I imagine some subjects will be easily identified because of the things they have (e.g. this has to be Justitia because of the sword & scales), but there are also many sculptures that don't have this imagery. It's possible some sculptures have "documentation" left behind by the sculptor, but it's improbable that they all do. Yet we are still apparently able to, e.g., write this for the Lady of Auxerre (figure from Wiki) even though its provenance is unknown:
The relatively small (65 cm high) limestone Cretan sculpture called the Lady of Auxerre (or Kore of Auxerre), at the Louvre Museum in Paris depicts an archaic Greek goddess of c. 650 - 625 BCE
So we can't identify which goddess it is, but we know it's a goddess. Why? Why can't it be a statue of a random Greek woman?
Related but narrower: How do we know this bust is Aristotle? which seems to imply that the statue of Clementia might not be Clementia at all, and the same goes for the Lady of Auxerre who might not be a goddess.