Check the website for the school and search for the class you need. There will often be an online description of class requirements, including the number of credits the class offers and how many credits you need to take it.
Call the school to ask about the credit requirements for a class. A large university will have its administrators divided up into smaller colleges or "schools," so you'll need the number for the proper department to get the most accurate answer for that class.
Talk to your adviser or guidance counselor if you have one. He can look up the information you need on appropriate credits or can guide you on where to find out.
Visit the instructor for the course. The teacher will be able to tell you definitively how many credits you need and any other requirements that are needed for the class.