If you are not sure which list, or lists, a particular subscriber is on you can search all of your lists at once using the individual's name or email address.
Searching all lists for a subscriber
Go to Marketing > Email Campaigns > Lists to see the "Manage Subscribers" page. Next, enter the subscriber's name or email address in the search field above the lists and click Search, as shown here:
If the subscriber is on only one subscriber list for the client, clicking search will open the subscriber's snapshot page. If they are on more than one list you will see a list of search results showing which lists the subscriber is on. From here you can click on a list name to open it, or click the subscriber's email address to open their snapshot page for a specific list.