The artist needs to confirm this. For example, many artists selling on Hicetnunc[1] will put their Tezos wallet address as a link in their Twitter bio. If you spot an artwork that looks like theirs minted by a different address, it’s likely a “copymint” (typically seen as worth no value).

There are other blockchain/crypto art projects that use the chain itself to store data; such as Autoglyphs[2], which is essentially a first of its kind generative artwork on Ethereum coded entirely within the Solidity smart contract. In this case there is no question about the authenticity; you simply look at that contract address and the tokens it initiated.

[1] - https://hicetnunc.xyz/

[2] - https://www.larvalabs.com/autoglyphs