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Depends on what kind of smarts you are looking for:

Quickness? Adaptability or rote responses? Knowledge? Trivia or domain knowledge? Ability to learn new ideas? Ability to analyze? Ability to memorize? Ability to improvise? Etc

You could probably ascertain surface level measures for those kind of questions in the course of a single conversation, but you’d only be seeing a sliver of their self in one conversation and it would be best to just ask the person “how smart do you think you are?” in some innocent way at an opportune moment in the conversation.