Drupal 7 Fields, when adding an existing field during content type creation, how to tell where else its used?
There's no UI for this anymore. The UI is in admin/reports/fields (h/t Clive)

Here's a good stack about how to find this info with drush:
Is there a listing of Drupal fields?

Or you can grab it from the database:

SELECT entity_type, bundle FROM field_config_instance WHERE field_name = '$MY_FIELDNAME'