There are certain words that appear in the results list of items in Discover@MU that constitute primary sources. The most important of them is sources, but there are others.
Once you perform a search in Discover@MU, you can click "Shoe More" under Filter by Format. Here you can check the box for Primary Source Documents.
In the advanced keyword search, you can put your search term(s) in the top box, and in the second box, put the word(s) you're using to locate primary sources in the next box, and change the drop-down menu to Subject:
Keywords that will help you find primary sources:
Depending on the period being studied, it can also be helpful to limit your search by publication date.
More keywords identifying primary sources in Library of Congress Subject Headings
These are just a few examples. Look at the subject headings for these records and read the descriptions to have a better understanding of what keywords to use to find books with primary sources within them or that are primary sources.
Voices of Early Christianity by 
Women in Ancient Rome by 
In Our Own Voices by These are only a few examples.
