If you do a quick google search you'll find thousands of sites offering IQ tests, if you do each one of these tests you'll end up with at least a hundred different IQ scores. There are also nationally recognised forms of IQ tests though they tend to cost you a few hundred dollars. So how can you tell how accurate they are? Well let me begin by explaining how they work.
First off IQ stands for Intelligence Quotient which basically means a measure of intelligence. Secondly there are a number of different types of intelligence recognised by most psychologists these are:
1. Linguistic - This concerns how good you are with words, i.e. writing, reading, telling stories or doing crossword puzzles.
2. Logical-Mathematical - Relates to patterns, arithmetic problems, strategy games and experiments.
3. Bodily-Kinesthetic - Things such as dancing, arts and crafts, athleticism and so on.
4. Spatial - This is about images and pictures. People with this kind of intelligence tend to be good at reading maps. It also relates to things such as drawing, building things, jigsaw puzzles and daydreaming.
5. Musical - Being able to identify notes and tunes others may not pick up on. Ability to play music.
6. Interpersonal - Involves communicating, understanding others' feelings and motives and leadership qualities.
7. Intrapersonal - Awareness of one's own feelings and motives. People with this kind of intelligence tend to be a bit shy.
These seven are the most recognised types of intelligence, though there are two more that were added later and that are still under debate amongst some people. These are:
8. Naturalist Intelligence - Ability to discriminate among living things i.e plant and animals, sensitivity to other features of the natural world i.e. clouds and rock configurations.
9. Existential Intelligence - Sensitivity and capacity to tackle deep questions about human existence, such as the meaning of life, why do we die, and how did we get here.
These categories were developed by psychologist Howard Gardner in the early 1980's.
Okay, now on to how IQ tests work. Most tests you come across will cover a few of the intelligence types mentioned above, usually linguistic, logical-mathematical and spatial as well as adding in some factual or general knowledge questions. Other tests focus on just one of these categories.
The test is set out as a number of multiple choice questions, sometimes they have have a time-limit on completing the test. Once you have answered all the questions or you have run out of time you will be given an IQ score based on the number of correct answers given.
Now, how to interpret this score. 100 is average, it has always been average and will always be average. This is due to how tests are standardised, which is to say before a test is published they give it to a sample group (of say 10 000 people) and take the average score achieved then make that average score equal an IQ of 100.
On most tests they'll also give a written statement of your intelligence, i.e. Below Average, Average, Above Average etc. This correlates to the number of standard deviations away from 100 your result lies and on which side of the scale you are. These standard deviations are also unchanging categories, they are:
Below 55 = Mentally deficient
55 - 70 = Borderline deficient
70 - 85 = Low intelligence
85 - 100 = Average
100 - 115 = Above average
115 - 130 = High Intelligence
130 - 145 = Superior intelligence
145 - 160 = Gifted
160+ = Universal genius
So back to the first question, how can you tell how accurate they are? That depends on the number of intelligence categories they test, what geographical culture they are aimed at (different cultures may have different knowledge when it comes to factual questions as well as recognition of objects. This is called cultural bias though I won't go into it now) and how many people were used to standardise it.
So are there any decent free ones available? Well in my experience avoid "tickle tests" unless your just looking for a meaningless ego boost, I don't believe these tests have been standardised at all, or at least not very well. The most trustworthy test I've come across is at http://www.highiqsociety.org/iq_tests/ though don't get disheartened if you don't do so well, remember there are several categories of intelligence it doesn't measure.