Most Helpful Guy
I have worked with brilliant people in the Art, Music, and Science worlds who were not very articulate.
For some years I wrote scripts for Educational films on Mathematics, Computers, and such technical subjects. The experts who supplied me with the information commonly found it difficult to describe the concepts in a visual way, and were often surprised ideas on advanced maths could be presented very simply in visual form.
I also worked extensively with Special Needs children who had communication problems.
Such children included autistic, dyslexic, severe physical or mental problems, hyper active, etc. All of them requiring special education because they could not be taught in normal schools, and commonly dismissed as unteachable, yet some of them extremely capable when taught how to express themselves in ways suited to their abilities
Regarding 'normal people', you may well judge them by their accent, and mannerisms. It is a common assumption that someone who is uneducated and has a common accent is not intelligent.
Miles Davis said he could tell if a musician could play well by the way he stood.
There are different types of intelligence
The only way you can judge a persons intelligence in any meaningful way is to see them working or acting in the field that they are familiar with, and this assumes that you are also familiar with what they are doing2