Several ways in the Project panel.
- the Preview area at the top of the panel shows video and audio usage. The little disclosure triangle there opens a popup menu showing each place it's used. Clicking an entry in the list brings that sequence to the front and selects the track item.
- In the list view, you can display Video Usage and Audio Usage as columns. To select which columns are displayed, open the flyout menu in the upper right corner of the panel and select Metadata Display..., then search for "usage." Note that columns these usuallyshow up toward the middle of the other columns. In CC and CS6, these usage numbers are interactive just like in the Preview area
- in CC and CS6, the Icon view shows a "badge" in the lower right corner if the clip is used in any sequence with the project--a film strip icon for video usage, a waveform icon for audio. Again, clicking the badge opens the same interactive popup menu mentioned above.
In a related vein, if you're working in the Timeline and want to find the project item that a track item is from, right-click and select "Reveal in Project." As of CC, the same feature is available for finding the sequence by right-clicking the sequence's tab.
Also new to CC, PPRO has duplicate frame markers that indicate if the same segment of a clip has been used more than once. You enable this through the Timeline Display Menu accessed via the wrench icon near the upper left corner of the Timeline panel.
More info than you asked for, but no extra charge ;-)