SDK question: Detecting import in the Read

Hello all,
My current method for detecting import in the Read function is to check if a Required attribute is set to it’s zero value. So for a string attribute that looks like:

if d.Get("required_attr").(string) == "" {
  // We are in an import, get the required attributes from d.ID()

However, what if I don’t have even a single Required field? How can I detect that the Read is being used for an import?