If a scientist on the other side of the world dropped an apple, it would fall to the ground. If someone dropped an apple a hundred times in a row, it would fall to the ground a hundred times in a row. Therefore, we can say with some certainty of truth that a force called gravity is in effect. We observe something happen, and that something is repeatable by others with the same results. That is the nature of scientific investigation, in a nutshell.
Science can discover truth about observable, repeatable phenomena.
Other avenues can also be used to discover truth. The legal system uses the premise that if many witnesses testify to the same event, then that is solid support for the truth of that event. Logic is the science of right thinking, and can help us determine further truth through the process of deduction, if we start at a truth and don't make a wrong turn along the way. This was also a highly-regarded vehicle in the search for truth. In a way, science is the least well-equipped vehicle to discover certain truths.