I recommend you develop a relationship with your son's teacher, to learn what you can expect in terms of homework for your son, and to see if the teacher will share his/her email address for follow-up when you're unsure if your son is being truthful about the assignments. Some teachers also set up websites (or use third-party services such as SchoolNotes.com) for hosting information about their classrooms, including homework assignments, curriculum notes, test dates, etc.
My daughter's teachers (from kindergarten through second grade) have been very willing to provide me their email addresses, and have been very responsive whenever I have sent inquiries via email. They've also taken time before and after class to discuss any concerns I've had. Back to School Night is another good time to talk with your son's teacher about classroom and homework expectations.
If you aren't able to go to school and talk to your son's teacher in person, call the school office and leave a message for the teacher, or mail a letter to the school, to the teacher's attention. If you don't have the school's contact info handy (and if the school is located in the United States), you can research it in SchoolFinder: http://www.education.com/schoolfinder/
I'm also including below some additional resources that I hope will be helpful to you. Good luck!
Some schools do have websites where they update what assignments children have been given. If your school does not have this facility then build a relationship with his teacher. Perhaps you can agree to have a homework diary where he writes down all homework and you sign to say that he has completed it and the teacher signs to say they have received it? Or chat with the other mothers at the school and see whether their children have been given homework assignments.
Every year our school provides the students with a planner. Each day the students wirte what they have done or what needs to be completed for homework. i am aware of when tests or quizzes will be and able to study with my child accordingly. The teacher signs the planner and even lets me know what kind of day my child had. This is a great deterrant for bad behavior sinse mom sees the planner. My job as parent is to check the planner every night and sign it. This is a great way for the children to become responsible and a fantastic way for parents and teachers to communicate daily. When or if either I or the teacher needs anything or wants a call or conference we write it in the planner. Awesome communication. If your school does not have a planner system ask for one, or stop by your Walmart or Target and pick one up. You can even use a composition notebook. Anything to be able to write assignments down and have teacher sign. The teacher should be very pleased to know you're taking an interest in your childs school life.
In this day and age most schools have a homework board. Large schools usually have one of their own. The small school use somethng like "Homeworknow.com. Ask your school principal where his/her school's can be found.