Fluid intelligence is the ability to reason. A person with high fluid intelligence is able to play around with unfamiliar concepts, quickly gain an understanding of them, and then use that understanding to solve problems. IQ tests assess this trait in a variety of ways. One example of a fluid intelligence question is as follows:
"A farmer has a chicken, a fox, and a bag of grain. He must take all three across a river in his boat, which is only large enough for one item. If left alone, the fox will eat the chicken, and the chicken will eat the grain. How does the farmer get all 3 across the river?"
To solve this, the participant must use fluid intelligence to work within the unfamiliar constraints imposed by the puzzle.
Crystallized intelligence refers to the knowledge one has accumulated during her life and the extent to which she is able to draw on skills and knowledge to reason and complete tasks. IQ tests typically test a participant's crystallized intelligence level by asking her to complete general knowledge questions and vocabulary tests.
Literacy is measured by strength of spelling, grammar, vocabulary and comprehension skills. This is tested by asking participants to supply synonyms or antonyms for particular words, or having them read a complex paragraph and answer questions on it.
Memory can be short term or long term. Short-term memory is the ability to retain information for brief periods of time. This can be tested by presenting the participant with a series of numbers and then asking him to repeat them, sometimes in reverse order. People with strong short-term memories can typically hold more numbers for a longer period.
Long-term memory is the ability to recall information which was learned at some previous time. Long-term memory tests are typically separate from IQ tests and may require a participant to learn a poem or list of facts for recitation at a later date.
Quantitative intelligence is the ability to understand values and quantities. People with high levels of quantitative intelligence tend to be extremely good at mathematics. A key quantitative skill is mental arithmetic and the ability to manipulate values and quantities using mathematical operators. IQ tests use arithmetic and percentage based questions to test for this.
Visual processing is the ability to visualize and manipulate patterns and shapes. One way this can be tested is by presenting the subject with a complex shape and asking what it would look like if inverted or flipped around.
Listening skill (or auditory processing) is the ability to tell apart different sounds. People who can play a musical instrument "by ear" generally have extremely strong auditory processing skills. One way this can be tested, is by playing the test subject a piece of music and then, after a few moments, playing the piece a second time and asking if the two pieces are identical. A person with strong auditory processing skills would be able to discern if any of the notes are different.
Processing speed is the ability to perform tasks under pressure. IQ tests measure this, not through specific questions, but by imposing a time limit on the test itself.