user1051: Let's say I have the asset string representing an NFT. How can I verify that a user signed in with Nami wallet owns that NFT? Is there some sort of signature they can provide?

user12: The blockchain stores the proof that a particular NFT belongs to a wallet. Whoever controls the wallet own the NFT. Wallet control is determined by access to its private key(signing key). If someone presents a text or number signed by wallet private key then he proves that he controls that wallet.
This is how websites are being connected to wallets.

user4422: This depends a bit on your use case.
If you just want to look it up on the blockchain: You can query the node with cardano-cli query utxo --mainnet --address <address-to-check>. This gives you all UTXOs at the address and the value and NFTs contained in it.
If you want a smart contract to validate that the user has the NFT: The UTXO with the NFT would need to be part of the transaction and then sent back to the owner. Then some validator can check the context of the transaction if the NFT is included.

1051: <p>Let's say I have the asset string representing an NFT. How can I verify that a user signed in with Nami wallet owns that NFT? Is there some sort of signature they can provide?</p>
 12: <p>The blockchain stores the proof that a particular NFT belongs to a wallet. Whoever controls the wallet own the NFT. Wallet control is determined by access to its private key(signing key). If someone presents a text or number signed by wallet private key then he proves that he controls that wallet.
This is how websites are being connected to wallets.</p>
 4422: <p>This depends a bit on your use case.</p>
<p>If you just want to look it up on the blockchain: You can query the node with <code>cardano-cli query utxo --mainnet --address &lt;address-to-check&gt;</code>. This gives you all UTXOs at the address and the value and NFTs contained in it.</p>
<p>If you want a smart contract to validate that the user has the NFT: The UTXO with the NFT would need to be part of the transaction and then sent back to the owner. Then some validator can check the context of the transaction if the NFT is included.</p>
