How can I tell if an attachment is internal or not?

I’m using ComponentAccessor.attachmentManager.getAttachments(issue) to get a list of attachments on my issue. Some of these attachments are internal and some are visible to customers in our Service Desk portal. How can I tell which is which? In the Attachment objects I get back from the call, I see no way to discern which attachments can be seen by external users and which can’t.