The best way to assess exposure to radon is by measuring concentrations of radon (or radon decay products) in the air you breathe at home.
The first step is to test your home for radon and have it fixed if it is at or above EPA’s action level of 4 picocuries per liter. You may want to take action if the levels are in the range of 2-4 picocuries per liter. Generally, levels can be brought below 2 pCi/l fairly simply.