Confirm that you have your anti-virus software up to date and your firewall up. This will protect you from websites that are trying to harm you computer. Avoid any websites that will offer results only after you have paid a fee -- this is the sign of a scam.
Type in terms such as, "free IQ test," or "free IQ quiz" to render the results you are looking for.
Use more than one search engine with different terms to help you gather more results. Search engines give results in the order of the most popular. If you notice a certain link that is consistently first, the website is probably legitimate and can be trusted.
Take more than one free IQ test to determine the level of accuracy. These tests are not standardized. While the tests are not necessarily fake, you should not rely on only one test as a source to provide an entirely accurate estimate of your intelligence.