There are many different levels of intelligence and several levels of stupidity, although dumb comes off as a single phenomenon to the observer.
We generally assume that someone is intelligent if they are like ourselves or if they seem to know more than we do. This is a highly deceptive, defective way of judging intelligence. I remember being in a “spot of bother” academically at one point in college. When I talked to a history professor about it, he asked me my grade in his class. I told him and he said he assumed if I could do well in his class, I should be doing well generally. I thought: should that really be the standard?
Intelligence is like any other important element in society, it is culturally and politically defined, often to confer advantages on people who possess some aspect of it, rather than general intelligence. Colleges reward the ability to think in abstract terms, to pull concepts from their context and to have the ability to analyze and manipulate those concepts and ideas. That doesn’t mean that this is the ultimate skill of the intelligent person. It means that there is an assigned value, good grades, to being able to do that. We assume that people who can handle these tasks are therefore capable of learning and developing, using those skills and life experience. This assumption is often wrong.
We ascribe lesser value to people often by assuming that what they say or write is stupid. In other words, what we wish to do is protect or elevate our view of ourselves, so we downgrade a lot of what we hear or read, deliberately misunderstanding it and not taking the time to correct our own thinking. Conversely, we assign great value to what we say or greater value to someone to whom we have granted some superior status, like a Nobel prize winner, etc.
I could go on and on…but generally what I wish to say is that we don’t have good, universally applicable standards of intelligence and our understanding of the concept is flawed. It is much easier to know someone is not intelligent than to surmise the opposite. In my work, I assume that the people with whom I come into contact are intelligent and that there is likely much more to their capabilities than what I see before me. This applies to people who do not have degrees and who work in fields where the mind is not pushed toward what are assumed to be higher conceptual thinking. Everyone reaches a point or a subject matter that can make them appear to be dumb. Likewise, so called menial jobs or ordinary functions, like running a small business, require the use of the mind and application of intelligence on a regular basis.
Here is one measure of intelligence that I was considering just yesterday: the ability to learn and then to know more than what is being taught. Someone who has not developed their intuitive capacities is, to me, not very smart. If all one can do is to study a text book and then follow “the rules” without bringing more to the given task, that person is inherently not intelligent. Being quick, learning a set of tasks and following a prescribed course of action is one sign of minimal intelligence. Being able to see beyond a limited set of instructions and being able to recognize surprising or unknown factors is a sign of higher intelligence.
The short answer to your question is that you are never likely to know the full intellectual capacities of people around you. Some are highly limited (and they will show this) and others have aspects that cannot be revealed in ordinary day to day contact. People can be intelligent and repeatedly do dumb things, repeatedly make the same mistakes. The best course is to not apply limits to what you think other people can do, but rather to be open to the possibility that they are much more than what you are able to perceive immediately. In speaking or writing with others, call on them to show you there inherent intelligence. If they aren’t, in most cases you won’t have lost anything by being open to more in them.