You can't see radon, but it's not hard to find out if you have a radon problem in your home. All you need to do is test for radon.

The amount of radon in the air is measured in "picocuries per liter of air," or "pCi/L." There are many kinds of low-cost "do-it-yourself" radon test kits you can get through the mail and in hardware stores and other retail outlets, but you need to make sure you follow all the recommendations about placement and maintaining closed house conditions. If this is not done, your test results will not be reliable.

If you are buying or selling a home, hire a qualified radon tester to do the testing for you.

There are Two General Ways to Test for Radon:

SHORT-TERM TESTING:

• The quickest way to test is with short-term tests. Short-term tests remain in your home for two days to 4 days, depending on the device. "Charcoal canisters," "alpha track," "electret ion chamber," "continuous monitors," and "charcoal liquid scintillation" detectors are most commonly used for short-term testing.
• If you need results quickly, however, a short-term test followed by a second short-term test may be used to decide whether to fix your home.

• Long-term tests remain in your home for more than 90 days. "Alpha track" and "electret" detectors are commonly used for this type of testing. A long-term test will give you a reading that is more likely to tell you your home's year-round average radon level than a short-term test.
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• Long term testing is a great way to keep up with the radon levels in your home -- very important if you are monitoring levels between 2 & 4 pCi/L.

What Your Test Results Mean

The average indoor radon level is estimated to be about 1.3 pCi/L, and about 0.4 pCi/L of radon is normally found in the outside air. The EPA's recommendation is to fix homes with more than 4 pCi/L, and to consider making corrections to lower the level or radon for test results between 2 pCi/L and 4 pCi/L. U.S. Congress has set a long-term goal that indoor radon levels be no more than outdoor levels. While this goal is not yet technologically achievable in all cases, most homes today can be reduced to 2 pCi/L or below.

If your living patterns change and you begin occupying a lower level of your home (such as a basement) you should retest your home on that level.

In a nutshell: The EPA's recommendation is for your average indoor radon levels is below 4.0 pCi/L and below 2.0 when possible. 

Radon Testing Protocol - What you need to know if your home is being tested