How do you know when a text box has an auto-complete?

I was working with TFS Build when I ran into this screen:

I went to Windows Explorer to get the entire path to CTRL-C/V into the text box when I realized that the text box had an auto-complete feature.  This saved me the time to switch between Windows Explorer and TFS, which was good.  However, outside of the documentation, how would I know that the text box has auto-complete turned on?  Drop Down text boxes have a down arrow by convention, multi-lists have a vertical bar, but there is nothing for auto-complete.  When Google introduced auto-complete, it expected a search phrase so they did not include any visual clue:

I am wondering if there should be some kind of visual clue that the text box auto-completes?  Perhaps a red-arrow in the far-right of the box? Anyone have any ideas?