I have not formally studied physics but am interested in quantum physics. I have studied calculus so I know a little bit about mathematics in case the answer requires it. My question is, when a physicist conducts experiments at the quantum level, how do they know that what they are "seeing" is what they thought they were expecting? To fine tune my question, how does one know for sure that a proton is composed of quarks held together by gluons? What if it is entirely something different, but because the physicists were expecting one result of the experiment, they were prejudiced toward believing that the results confirmed what they were looking for?