Olive Garden’s corporate owners wont do NFT’s of these low quality images of their locations and pretend its ownership of one.

So because their wouldn't be a collision it doesn't matter that the non-owner minted these earlier.

Even if they did, to be ironic, the social proof from that official issuance, would attract all the value, ignoring these. Which has also happened before in the NFT space.

How would I know that this site was not operated by the corporate owners without the explicit disclaimer the creators put in?

Why do you think it matters?

If the owners gave validity to this collection later, it may accrue actual value despite being incapable of being a claim to any building or land or franchise.

How would I know "if the owners gave validity to this collection later"

Either because other people want it and placed high bids, or there were a spike in other people wanting it, or offchain PR from trusted sources.

You have to check your own collection, or collections you find interesting. There’s not going to be a push notification.

Whether its a collection of polaroids or a collection of nfts its the same in this regard. (except you could make your own tool to know about the bids and volume in real time with the NFTs, and the chain of prior purchases and prices are built in, compared to physical goods on ebay or trying to aggregate and archive craigslist where you also wouldn't be assured of those pieces being actually part of the desired collection)