First, navigate to the "Manage Contact Fields" page. Do this by clicking the "Contacts" tab at the top of your page and then the "Manage Fields" button:
You'll be brought to your "Manage Contact Fields" page:
Notice in the example above, there are 2 uses listed for the contact field "Age". By clicking on that link, we can see that the contact field "Age" is being used in two flows:
Learn more about managing your contact fields here.