This varies by school. Some will allow you to skip prereqs with instructor or advisor permission, some make that almost impossible. Some may allow you to audit the course but not take it for credit.
Honestly for the larger CS classes I took, there were enough people in the class that I doubt anyone would have noticed a person slipping in to a back row seat and just quietly watching. More than once I sat in on classes that I thought I might want to take, to get a better sense of the content and the professor's teaching style.
Honestly for the larger CS classes I took, there were enough people in the class that I doubt anyone would have noticed a person slipping in to a back row seat and just quietly watching. More than once I sat in on classes that I thought I might want to take, to get a better sense of the content and the professor's teaching style.