It's true. Sometimes, people are going to mark your email as spam. It happens to the best of us. Our jobs as communicators are to ensure that these folks aren't included in our email messaging in the future, as to keep our complaint rates (spam) as low as possible. So, where do you find the list of recipients that have marked your email as spam?
1) On a Campaign by Campaign basis, you can look under the Campaign Members related list in Salesforce and look/sort for all the Campaign Members whom have a check mark in the checkbox field labeled "Complaint".
2) On an organization-wide basis, you can run a report in Salesforce for all Campaign Members that have a check mark ("true") in the checkbox field labeled "Complaint".