It depends on your child's age, but your first stop should be your child's doctor.  Speak to them about what is worrying you, especially behaviors that might be a sign of ADHD:  combinations of hyperactivity, inability to stay focused on a task or finish a task, inability to do homework, destructiveness, moodiness, sleeplessness, or any other behaviors that give you concern.  If your child is school aged or in pre-school, your school district should have a program to give the child an assessment.  School districts are required to do so (although it may take some time to get the test set up).  Collect "evidence" from the child's teacher -- examples of in-class behavior that the teacher has noticed that's indicative of ADHD.  Your child's doctor may also suggest you see a child psychologist who can give the child the right tests to assess if they have ADHD.