Custom field

It depends on the format of those fields.

Assuming that it’s a custom field that contains just a single URL and nothing else, you can configure BLC to check them like this: go to “Settings -> Link Checker”, switch to the “Look For Links In” tab and enable the “Custom fields” option. Click “Configure” and enter then name of the field in the box that shows up. If the field contains HTML code, enter html:field_name_here instead.

If the fields contain something like JSON or XML data, or if they’re stored in a custom database table, there is currently no way to search them for broken links.