Final Cut Pro X has a nice way to help you figure out which clips (or part of a clip) have already been used in your timeline. It is more a workflow than a trick and goes like this:

  1. Favorite the clip (or clip part) you want to use in your timeline by pressing the F key with the desired section selected. A green line will appear at the top of the thumbnail.
  2. Append the sequence to your timeline.
  3. In the event browser, filter the view by favorites and you will see the list of everything you used so far.

As a rule of thumb, always take the habit of selecting clips you want to use as favorite and discard sections you dont want to use as Rejected (backspace). These filters take a few hours to get used to but provide an incredible productivity gain.

Edit: SafeSolvent also suggested to use the Timeline Index which has a search box and display the clips in their order of appearance if you dont need to look at the clip.

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