What is the best way to test for radon gas?

Answer 1

Charcoal type radon gas test kits are very reliable, whether in a canister, bag, or vile. These are readily available and inexpensive.

There are basically 2 types of radon testing methods short term and long term measurement devices. A short term device provides a quick snap shot of the radon level in a building. They are usually placed for 2 to 7 days. Long term testing will give you an average level over a much longer time period. Radon exposure risk is based on long term averages. So accepted protocol is start with a short term test, if the level is above 10pCi/L you should fix the problem. If the level is below 10pCi/L you can either retest to verify, or do a long-term test look at the long-term average.
The EPA's action has been set at 4pCi/L but they also recommend that if the level is between 2 and 4 that you should consider fixing the home, since radon is a radioactive gas.

Answer 2

Hardware stores sell a one-time test for about $15. I bought an electronic tester that checks constantly, however the reading was about the same all year long. A one time test is probably good enough. Make sure you read the directions carefully.