In California and most other states in the USA, at public universities (and sometimes private universities), it is possible for anyone who is not affiliated with the institution to walk into a classroom and learn for free. In small classes, it is polite to show up 5 minutes early and ask the class instructor if your presence is welcome (the answer is almost always yes); in large lectures no such introduction is necessary.
Is it similar in Japan and S. Korea? If I want to visit a class at the university, what sort of introduction should I make to whom, and how do I know which classes are more 'open' to the public in this sense and which ones are more private?