Summary:
There is no easy way to identify the usage of custom fields across projects.

Suggestion:

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:

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