We hardly find any persons of good sense save those who agree with us.

Agreement is the quiet standard by which most of us judge intelligence. Someone who sees things as you do seems clear-headed, and someone who does not seems to be missing something, and the assessment feels like an observation rather than a preference. This is how a person ends up surrounded by sensible people who all happen to think the same thing.

Take someone you have filed under not very bright and look at what they believe that you do not. Find the strongest version of their reason, state it in your own words, and see whether it was really stupidity you were detecting.