By the on-chain transfer log. For a QRC-721 item, the recipient of its most recent transfer holds it now; if that recipient is the zero address, it was burned and has no owner. A QRC-1155 item can be held by several addresses at once, each with a quantity — the holders list shows them all.
What is the mint?
The mint is the token's first transfer — created from the zero address (Q0000…0) to its first owner. Everything after it is a regular ownership change.
Why is the image only a link?
An NFT's artwork lives at an off-chain URL that whoever deployed the contract chose. Quantascan reads the item's metadata (name, description, attributes) but never loads, hosts or checks the media it points to, so the image appears only as a link you can choose to open. The on-chain provenance — who owned it, and when it moved — is always shown.