What is the evidence is there that there are atoms? Why do we think they exist? There are several lines of evidence. By the way, the image above is of Mr Potato Head from Toy Story™ and is used without permission.
The study of how we know things is called "epistemology." What does it mean to say you know something is true? Can you be certain of anything? It's actually a hard question and one that has occupied philosophers for millennia. We will base knowledge on what we can measure and we will never really be certain.