All good answers from my colleagues and my two cents is this. Highest volume will change from month to month and year to year.
Unfortunately, there's no easy way to answer that question because there's no standard. Marianne makes a great point of testimonials.
When you interview agents, see what their clients are saying about the service they provided, the results they produced and whether or not they would work with them again. Do those values align with yours? Those are the important questions.
Good luck to you, Megs.