Choose the Detail view in Project Media.
You'll see how many times each media is used in the project.
If you load a whole folder in Project Media, you'll see instantly what's being used in the project and what is not.
Thanks for the replies. Sorry for the late response.
Actually it's not solved. What does the Usage count actually mean? I have clips that AFAICS are used only once in the project as it stands, but are shown as being used up to 13 times, in one case.
Does usage count refer to how many times the clip has been dropped into the video, or copied/split etc, or what?
Yes, the usage will indicate how many individual events on the timeline are linked to that video file or audio file, photo, etc.
It may surprise you, but for most video clips you will see usage = 2 even if it appears only once on the timeline, as the video file is being used by the video event *and* by the associated audio event as well. That counts as two events.
(Or at least that's how I remember it -- I'm not at a computer with Vegas installed right now...)
Well, to address your original question, the only thing that matters is if the usage count = 0 or if it's > 0. 0 means not used. Any other number means it is used.