Do with a file open for ‘add’, if it’s submitted via another client

I have always used the p4 where command for this. Here is its description from the built-in help:

where -- Show how file names map through the client view

p4 where [ file ... ]

    Where shows how the named files map through the client view.
    For each argument, three names are produced: the name in the
    depot, the name on the client in Perforce syntax, and the name
    on the client in local syntax.

    If no file is given, the mapping for '...' (all files in the
    current directory and below) is shown.

    Note that 'p4 where' does not determine where any real files are.
    It only computes where they should be according to the client view.

From the "View" menu, select "Workspaces". You'll see all of the workspaces you've created. Select the workspaces you want to delete and click "Edit" -> "Delete Workspace", or right-click and select "Delete Workspace". If the workspace is "locked" to prevent changes, you'll get an error message.

To unlock the workspace, click "Edit" (or right-click and click "Edit Workspace") to pull up the workspace editor, uncheck the "locked" checkbox, and save your changes. You can delete the workspace once it's unlocked.

In my experience, the workspace will continue to be shown in the drop-down list until you click on it, at which point p4v will figure out you've deleted it and remove it from the list.