In my past experience, it really depends on Professors.
Some of the classes are HUGE, it's almost impossible to tell who is in the class and who is not.
Most of CS classes are in the size of 40 to 50. For the most beginning few weeks, there will be no seat for some students, because students are trying to add (hence 旁聽 is impossible/hard).
What class are you trying to 旁聽? why not list them here?
You will need to talk to the professor. But if you ask me, I won't suggest to do this. Teacher job here earn very little money, very little, it is not nice to just sit in to the class and just listen there but not paying. It is not fair to the teacher. Unless you talk to the professor, and he or she does not care. I've never just sit in to some classes without register the classes for my college and master. It is expensive, so make sure you really are going to take the classes.