You can take an IQ test that is very similar to Mensa's tests at MensaIQTest.net. The test is free to take, includes 60 questions and has a time limit of 40 minutes.
The average IQ is 100. Mensa accepts new members into the organization if the applicant has scored an IQ test result in the 98th percentile, or top 2 percent, of the population. Typically this means you can be considered for Mensa if your IQ is 132 or above on a Mensa IQ test. If you score 132 or above on a test with a similar difficulty to the Mensa tests, such as the test at MensaIQTest.net, this suggests that you may have the required skills to become a member of Mensa.