You are totally right that it is incredibly hard to evaluate skills during a small in person interview. Interviews are about 40% evaluation of skill set and 60% culture fit. A lot of research show that a bad personality fit in a culture does more to damage output of a group than having a single stellar employee. By the time someone interviews you generally can get a sense for their skill set before they interview via their resume and then asking them to submit either previous work samples or by creating a skills assessment test. Other than that... it is really hard to evaluate people's inner skill set, work ethic etc.