There are 2 ways to find out if main campus (University Libraries) owns a title.
For print titles, you will see "Held by: University of New Mexico Libraries" at the top of the record underneath the title. Under the "Availability" tab, you will also see this as well as the holding library and call number.
For electronic resources, you will see "Held by: University of New Mexico Libraries" at the top of the record underneath the title. To access the electronic resource, click on the "Availability" tab and then click on "UNM Main Campus Libraries". This will take you to University Libraries instance of WorldCat where you can access the title.