But you may have noticed that Facebook has recently changed so that if you go to the Page, Facebook requires that you Like the Page before being able to access the Interest Lists options. That makes it harder to spy on them.
When you go to a Page, you can put them on an Interests list but the menu is only available after you have clicked Like
If you want to put them on an Interest List without Liking the Page first, you will need to do that from the Interests Lists area found at https://www.facebook.com/bookmarks/interests
You can either create a brand new Interest List of Pages, or add Pages to an existing list. To add pages to an existing list, click on the list and select Manage List and Edit List in the upper right corner.
Now select the drop-down menu to include all Pages and search for new Pages to add to your existing list. You will have to type them properly – Facebook is particular about the search.
Take a look at the video to see exactly how it works.
I also have a good resource on how to spy on your competition here: http://www.socialmediaexaminer.com/research-competition-on-facebook/