how do you verify your knowledge in a subject area for grad school admission while you have not been enrolled in regular class?
What you want to do will limit your choices among schools.  Where I teach, we do allow provisional admission for people without the requisite undergraduate degree.*  Those students are required to take a series of foundation courses before beginning the "real" master's courses.  It is possible, although not easy, to test out of the foundation courses.  The provisional admission is revoked for the student who earns unsatisfactory grades (C or below) in the foundation courses.

So, you will need to look for institutions that allow provisional admission, and then check the details of the provisions.  

* Students must have some accredited undergraduate degree to be admitted; but it need not be in computing.