"Rabbi Alnekaveh told a story he heard once from an etrog-seller who used to bring Rav Eliyahu etrogim to check: "It is well-known that Rab Eliyahu could look at an etrog for mere seconds and immediately discern its advantages and defects..."
If a sukkah is built so poorly that it is not possible to sleep in it, some authorities hold that it is not kosher and that one may not even eat in it. However, in practice, we do not follow this view, and such a sukkah is nonetheless kosher.