Testing for learning problems is usually done by a psychologist. Ability is compared to your child's performance. When a child performs at least 1.5 years below his or her ability, he or she should be tested for specific Learning Disabilities. Usually the children who get missed are the ones who have more minor disabilities or children who perform at grade level. If you have a 9th grader who is capable of doing 12th grade work in math and science, but functions at grade level or slightly below in reading or writing, your child may have an undiagnosed Learning Disability, but may never be evaluated by the school system.
Most school districts focus on the children who are performing below grade level.
Advocacy groups, such as TASK, Team of Advocates for Special Kids, can answer any further specific questions which you might have.