You measure intelligence mainly in two ways, both of which involve tests. Take a standardized test administered by a professional, or take one of the many online tests. The former are more valid, but you have to make an appointment to see a psychologist or a school counselor qualified to administer and interpret the test. As of 2008, the most widely accepted test was the Wechsler Adult Intelligence Scale, according to socialsecurity.gov/disability.
Measure verbal intelligence with the WAIS verbal scales. According to career-and-test.com, these measurements include English vocabulary, comprehension, general knowledge and common sense, and simple arithmetic. Also measured is spatial intelligence, the ability to visualize and understand concepts. Finally, this part of the WAIS includes a measurement of memory, sometimes called "digit span." The subject memorizes a list of numbers and repeats them. This element also measures attention and concentration
Measure non-verbal intelligence with WAIS performance scales. These include completing pictures, arranging pictures so they tell a story, creating block designs according to patterns, matching shapes or sets of numbers and assembling puzzles. Puzzles are another measure of visual spatial aptitude. Performance scales also include matrix reasoning and symbol search. Some portions of WAIS testing are measured for speed and some are not.
For children and adolescents, there is only about a 50 percent correlation between intelligence and grades in school, so grades are not a good measurement. Intelligence usually doesn't change significantly after the age of 16. The Stanford-Binet test is appropriate for ages 2 1/2 to young adult. There are other standardized tests for children such as the Ravens Matrices. According to minddisorders.com, the Wechsler Intelligence Scale for Children is most appropriate for people 16 and 17.
Another way to test for intelligence is to apply to join a high-IQ group that offers its own tests. The oldest and largest of these is Mensa, whose activities are described at us.mensa.org. Tests can be found at mensa-test.com although the site says, "They may not be so accurate, but should be very fun and entertaining to do." Tests are also offered by the International High IQ Society, highiqsociety.org.