Radon levels in buildings can be measured relatively easily, but because levels vary from day to day (mainly due to the weather), the Health Protection Agency recommends a standard test over three months. Normally two small detectors are placed in a living room and a bedroom of a house respectively. Quicker 'screening' tests are available. However these have their limitations.
Radon is measured in units called becquerels and is quoted as a concentration in the air - i.e. becquerels per cubic metre.
There are several laboratories that are accredited to do radon testing (see the Health Protection Agency website for details). There will normally be a charge for providing the test.
The government and the Health Protection Agency recommend that radon levels are reduced in homes to below the action level of 200 becquerels per cubic metre. In most cases levels can be reduced by installing a radon sump.