First be logged as JIRA administrator and then click on the visible only to such user cog icon. Next perform the same action on the "Issues":

Scroll down the page and find "Custom Fields" link on the left hand side, then access it:

In order to see custom field usage perform "Show usage" action on a chosen custom field:

If a field is used, then you should see something like this:

 Please be advised that to get a view like on the screen above, by "used field" it is meant that it is insufficient for you, to only have an issue with an empty field displayed in it - a value must be placed into such field (or selected from a dropdown menu):

Otherwise you can expect a message like below: