forms - How to tell the user that an input box is autocomplete-enabled?

Is there a good graphical sign which tells the user that a particular field on the page is autocomplete enabled?

For instance, the field name says "Document author". When you start typing in that box, an ajax post-back is executed and a list of persons with similar names is provided. However, when the user enters that field , there is no sign that the box is somewhat special (even more - that he/she will not be able to enter a random name, but will have to pick from the list.)

So, I'm looking for a graphical icon that I could put in the background of the input box. I thought of a hellipsis (looks strange), downward arrow (confusing, makes user think it is a drop-down box), checkmark (makes no sense).

Answer

Show a downward arrow in the box that implies it's a drop-down. Then, perhaps, supplement it with some small help text. That's how Atlassian JIRA solves the problem and it works quite well: