There are two ways to do this:
A. Ask directly to the client what they thinks are the main four competitors. (Sometimes what they think is not the real competitor in regards to SEO ,that’s why it’s important to do the following B methodology.
B. In Agency Analytics go over to the left menu and click on SEO, and then on Rank Tracker, after this scroll down and identify a keyword important in which they are not in the 1st to 3rd position or that it has recently lost positions.Click on the Keyword and then select SERP. You will then be able to see the competitors ranking in first place. Make sure it’s not a listing such as Yelp or lawyers.com since we only want to use competitors websites.