How do we know U.S. elections are free and fair with accurate counting of the popular votes?

People speculate that votes are not properly counted -- even in the U.S. Have Gallup polls or well-respected independent entities tried to confirm U.S. election outcomes?

Update on 10/13/18: while the electoral college is relevant, I am looking for someone to address voter fraud and secret forces illegally counting the votes of the citizens or the electoral college inaccurately. My intended question pertains to this conspiracy theory.