You can tell if a wire story was already used by someone by looking at the DS or at the D column on the wire stories listing, on the List or on the Details directory style. On the following screenshot, the top graphics shows the D marker on the Details directory style, while the bottom one shows the DS marker on the List directory style.

The default marker appears as a blue rectangle in the rows of the stories that have been already merged at least once to the articles. If you hover over the rectangle, it shows a tooltip with the article name, merge date and time, and the user name.