When reviewing the respondents in Responses by Person, if you click on a particular respondent, the line will become gray. In this way you know which respondents you've already viewed. This is similar to read/unread email functionality, and lets you see which respondents you haven't reviewed yet.
You can also star your favorite respondents, so you can remember to return to them later. For more information read this article: How to Star Favorite Quotes/Respondents