How do I know for sure if my child has add/adhd?

Get an opinion. First step is to discuss your child's symptoms with your pediatrician. If she has the cardinal symptoms-inattention, impulsivity, distractibility, disorganization, hyperactivity, academic failure, behavioral problems-then a thorough neurodevelopmental assessment could determine the cause. Your pediatrician can guide you here. Adhd is a significant neurobiological disorder that should be addressed.
Good evaluation. Your child should have a good thorough exam and discussion of the problems that you are having. They should have hearing and vision checked. They should have testing, including questionnaires filled out by caretakers and teachers. They should have a discussion with a doctor to rule out other issues such as learning disabilities, depression, anxiety, conduct disorders, etc...