Is there a list somewhere of the "canonical questions" for general topics like the "has this food gone bad" one? I want to close something as duplicate, but I can't find the question it should link to.
This doesn't exist as an official feature. Meta sites tend to use a special tag for this called [faq], but it doesn't exist on the main sites.
There is a way to get close to it using SEDE. It is possible to write a query which finds questions that have been most frequently used as duplicate targets. It turns out that Glorfindel had already written one for Stack Overflow, so I copied it for our site: https://data.stackexchange.com/cooking/query/1630878/most-frequent-duplicate-targets.
These are not questions which are "canonical" by community decision. In fact, I don't think these decisions have ever been explicit, except for How long can I store a food in the pantry, refrigerator, or freezer?. Instead, people have gotten tired of similar questions, and written a more general one, and maybe mentioned "I hope we will be able to cover this topic now without the bothersome piecemeal questions", and the community started closing as duplicates afterwards. So they are questions which are used as canonical in practice, and the community has adopted them as useful resources.