It is often said that we can never tell what another person is thinking. This means that a person's thoughts are available, in a direct way, to that person alone. What we may believe is true about this claim is that we can normally conceal our thoughts from others. What does not follow, however, is that thoughts are essentially inaccessible and subjective in the sense of not being available in principle to scientific observation.
The behavior of people may give only a rough or a false impression of what they are thinking.
But, when they are speaking, it seems fair to say that, except under unusual circumstances, what they say is what they think: We know what they are thinking from what they are saying. In conversation, people express their thoughts, and even when they think in silence, their thoughts are potential utterances.