[throwawayaway12]

My experience has been as an enrolled PhD student in the US, so my thoughts may not completely apply.

I would just go talk to the professor teaching the class if you do not care about any documents saying you took the class. I have sat in on many different classes this way.

Things to note: -You may have some trouble on both extremes of class enrollment. Some courses require a minimum number of students to be enrolled for the class for it to be offered, so if there are not enough or bare minimum number of people taking the class you might experience some resistance. On the opposite end, for classes with long waitlists, you may not be able to sit in. -If you are not affiliated with the university, you may have trouble accessing course files that are locked behind a university authentication.

My advice if you want to go this route is to minimize your impact on the professor. Wanting special accommodations, such as special delivery of class materials or participating in homeworks/projects/exams may give some professors pause.