Read course syllabi from front to back before asking the instructor about class policies. If the answer isn't in the syllabus, the instructor will be happy to talk with you. Know the class policies in the syllabus. Although attendance is often not taken, especially in large lecture sections, you are responsible for all announcements made in class, in labs, and on class web sites. This includes written announcements made at the beginning of class, answers to questions from students, and any general observation that the instructor makes in class.
Many announcements spring up spontaneously from the flow of the class and dialogue in class.
You should rely on your own class attendance to make certain you don't miss anything. Come to class on time. Studies show that missing this part of class has a negative effect on a student's grade. If you have any problems with getting work done, with exam schedules, or anything that requires you to meet a deadline, it is your responsibility to notify the instructor in advance. Know when and where examinations are given and be there.