When intellectual disability is suspected, the assumption is that the individual will mainly have trouble with basic concepts, with communication, and with functional skills (activities in general). The way that adaptive behavior scales are used to measure intellectual disabilities is by interviewing parents, teachers, and even the student using the Vineland Scale II. This scale measures the three salient deficiencies mentioned before. This is a favorite scale because it comes with strategies and interventions already suggested. The final answer comes by averaging all the surveys given to the student's immediate support systems (teacher, parent, caregiver, etc). This is essential documentation needed to determine eligibility for ADA services.
IQ testing (Intelligence Quotient) testing is used by giving a standardized test. This test will hit specific areas that have mainly to do with problem solving. This is not problem solving as we think of in Math or Science, but general deductive and inductive abilities which lie at the core of what is known as "intelligence": the sum of all knowledge brought in by inferring and concluding. The overall ability in cognitive usage is compared to the rest of the population, and comes to a score that is also used as documentation for intervention and services. Stanford Binet is an example of these types of standardized testing.
As with everything, simple numbers do not tell the entire story. Other forms of assessing the possibility of intellectual disability include observations (empirical research), portfolios demonstrating the ability (or lackthereof) of the student, and the documentation of student behavior in small routine activities, communicative activities and concept-development activities.