Identifying unused files in Premiere Pro
Head into your Project Panel and check the columns you have available. You may need to scroll right, but look for one titled "Video Usage." 

If it isn't there, right click on the columns and click "Metadata Display..." Drop down the Premiere Pro Project Metadata menu and check "Video Usage." 

Now, using the "Video Usage" column, you can see how many times each clip is referenced in a sequence. Anything without a number next to it is not used.  A quick tip, if you have any audio-only files or used just the audio of any clip, you may want to also include the "Audio Usage" column and cross reference the two of them together when making your decision. Either way, if any clip is referenced in a sequence, Premiere should warn you before allowing you to delete it.