Few things that stand out for me when someone is intelligent: 1. The quality of their questions: they're able to get to heart of the matter very quickly throug

Few things that stand out for me when someone is intelligent: 1. The quality of their questions: they're able to get to heart of the matter very quickly through asking relevant questions. 2. Ability to be perceptive, seeing both something that's odd and something that's not there, such as the curious incident of the dog in the night time. 3. Ability to explain things at the right level of the audience: I'm a big fan of 'Things Explainer' for this very reason.