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Edit: I thought you said "determine" rather than "know", answered accordingly below. Of course they don't "know", that's not a helpful question.

The same way everyone does when trying to solve this problem systemically (vs individually), by making imperfect decisions and leaning on categories.

My UVC lamp was delayed. I think that's more important than someone else's "third month" bunker food, but what are you going to do.

A guess: it’s probably just heuristics based on things being panic-bought, high demand, suddenly high number of searches, etc. Obviously they don’t “know” some subset of essential items, that would be difficult, error-prone, and they would inevitably miss certain things which would lead to complaints and bad press.

At a high level, at least, anything in categories like "Groceries", "Health Care", "Personal Hygiene".