To find someone who has unsubscribed from a particular campaign click on your Reports tab and the find the campaign that you want to query. NewZapp will then show you a summary screen for that campaign and there is a tile at the top that shows you how many people have unsubscribed from that campaign.
If you click on this till you will automatically be taken to the Data tab where the drop down box underneath has been changed to show you the contacts who have been unsubscribed.
If you want to see if a particular contact has unsubscribed you can find instructions on how to do that here - Can I look up who's unsubscribed from my list?