I don't recall all that much beyond the basic plot. A human being is taken captive by a group of aliens and questioned. I believe it takes place on Earth and the aliens are a scouting party of some sort, though I am not certain on that point.
The aliens clearly have superior technology to humanity, and before they question the human they make it explicitly clear they will be able to tell if he is lying (with all manner of dire consequences to follow if he does so). I don't think they are psychic per se, but rather have technology that acts as an infallible lie detector.
They ask a series of questions (the specifics of them I cannot recall) designed to determine the strength/capabilities of Earth. The man answers truthfully, but is clever enough to answer the questions in a way that will give the impression to the aliens that Earth and mankind is a lot more powerful than it seems. I recall there is some internal rationalization/dialogue on why a particular answer is truthful.
The story ends with, as best I can remember, the aliens believe that attacking Earth would be a foolish and dangerous prospect, and the man is freed, secure in the knowledge he just saved the planet.
I'm really fuzzy here. I think I read it in a paperback anthology, not a sci-fi periodical. It may have been in an anthology that also had the short story "Heavy Planet", but I'm not at all sure about that point. I can say that I read the story in the late 1970s, and the style and plot of the story felt very "Golden Age" era sci-fi.