From xkcd: http://xkcd.com/263/
Question: How can we trust ourselves (or scientists) to know the truth about anything?
Answer: We look at the evidence.
Scientists back up their claims with evidence. If the evidence doesn’t fit the claim, then the claim is rejected and revised. New evidence can result in changes to long-held understandings about how the world works–it is the evidence that rules in the scientific process!
Through experiments and models, scientists test their hypotheses to learn more about how the world works.
Will we ever be totally certain about how the world works? Nope. But that just means that there will always be something to discover!