Final answer:
To identify the percentage of repeat visitors to your site, use the Behavior - New vs returning report. This report categorizes visitors as either new or returning, allowing you to easily compare these two metrics.
Step-by-step explanation:
To determine the percent of your site traffic that has already been to your site before, you would use the Behavior - New vs returning report. This report specifically breaks down traffic into 'New Visitor' and 'Returning Visitor' categories, providing you with insights into user behavioral patterns on your site.
The Frequency & Recency report can give additional details on how often users return and how much time passes between visits, but it does not provide a direct percentage comparison of new and returning users like the New vs returning report. The other reports listed, such as Interests - Affinity categories, All traffic - Referrals, and Ecommerce - Sales performance report, provide different types of data that are not focused on visitor recency or frequency.