...
In order to see custom field usage perform "Show usage" action on a chosen custom field:
If the 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 expct a message like below: