On the 13th of October 1917, near Fatima, Portugal up to 100,000 people allegedly saw the Sun dance about the sky, even though several billion people elsewhere around the globe did not notice the destruction of the Solar System and the Earth being hurled out into deep space. Clearly, if these people thought they saw this, they are deluded: no matter if there was 100,000 of them.
Scientists base their conclusions on observations with varying levels of certainty: they use these observations to build up falsifiable hypotheses and models (or theories, in the scientific sense). In addition to this, we can know that scientists generally know what they are talking about because science has brought in results unrivaled in any other field: i.e. science works, and has had exponential benefits to humanity.