i've found a better answer already.

I think it's better to say "sit-in" a class.

to share, welcome if any better idea. thanks for replying.

You will say that you want to "audit" a class.

Means that you will sit in the class, but not as a formal student. That means that you will only listen to the lectures, but will have no access to any written material nor will you receive a grade.

It is much better to take the class as a pass/no pass, credit/no credit so that you may receive all class material.  The beauty of that is that you will have no effect on your grade if you do not do well nor do you have to do all the course work.

take some psychology classes as a visitor student.

hope this can help