To see where a custom field is used

To see all items where a field is used

Click on Settings menu
Choose the Custom Fields tab
Click a tab with the entry type
Click on [i] in the Entries column

To see where items with a specific valid value are used

This functionality works in a similar way for articles, files, news and feedback, even though the example given here is for articles.

Log on to the Admin area
Click on Articles (if field is under Articles, or Files, News or Feedback if field is under, respectively, files, news or feedback)
Click on the [+] button to the left of the category drop-down

This brings up a list of all custom fields for articles (or files/news/feedback)

Select the specific field and value that you are looking for
Click on Search.

This brings up a list of all articles with that particular custom field value.

Bulk actions available depend on your user access.
For example if your privilege does not allow you to delete items, or allows you to only delete articles that you have created, then there is no 'Delete' action in the list of options. If your privilege does not allow you to update items, or allows you to only update items you have created, then this menu will not be available for you at all.