Summary:
There is no easy way to identify the usage of custom fields across filters/boards/dashboards
Ability to get Audit Log for Custom Fields, information on where the custom field is used across all the projects of the instance.
Note, this feature is offered in Jira Server: https://confluence.atlassian.com/adminjiraserver/analyzing-the-usage-of-custom-fields-1047552722.html

Suggestion:
Have a way to identify/admin custom field utilisation across filters/boards//dashboards

This can help when there is a need to migrate/utilise a new custom or system field instead of an old custom field.

There are some information that I believe could be present on admin custom field's page that could increase the admins experience (at least mine in this situation) like:

Number of issues with information (now, or at some point in the past) registered on this field;
List of projects these issues belong to;
Field created date;
If it is really custom or some system field identified as custom;

Workaround

For Filters, a user with the Administer Jira Global permission can go to 
YOUR_SITE_NAME.atlassian.net/rest/api/3/filter/search?expand=jql&overrideSharePermissions=true

-to view all filters and the JQL they use.