Show only used sources

I have a big library filled with all the (potential) sources that I thought I might need for my thesis.It turned out I didn't need them all so now there's literature in my library that I have not used in my document.
Since tot all of the sources are structured the way they need to be in order to bo quoted correctly I am wondering: Is there a way to find out which ones I have actually used within my document?That way, I could spare the work of formatting sources that weren't used at all.Help much mich appreciated!
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