Review miningSearch suggestions (autocomplete)
The completions the store offers in its search box. The most direct demand signal reachable without an ad account.
Suggestions are ordered by search volume. Which prefix length surfaces a phrase, and in what position, is a strong reading of how many people search it - and it needs no account at all.
This is the heaviest signal in rankcusp's popularity estimate. It is also one of the three sources the keyword pool is built from: terms people actually type, rather than terms somebody guessed.
It has two traps and both are silent: without the right storefront header the endpoint returns an empty list rather than an error, and it answers in XML rather than JSON. Both produce results that read like “nobody searches this”.