how do you verify your knowledge in a subject area for grad school admission while you have not been enrolled in regular class? If you want to apply for graduate school in computer science , you have to show that you have the prerequisite background knowledge . What are the available options ? Can I take a challenge test for credit in one subject area ,say ,algorithms provided that I have the required knowledge in this subject ? There is a man who managed to complete most of the undergraduate MIT computer science curriculum using MIT OCW . Can he verify his knowledge to the admission committee by taking challenge tests?Can I take the final exams that the university department offer to regilar students without attending to prove that I have the required knowledge for the graduate program ?
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.