The best way to find out if the soil beneath your house is expansive is to have a report done by a geotechnical engineer. In many housing developments, a Soils Report is prepared, but this requirement varies depending on the region or country.
A geotechnical engineer will drill soil brings on your site and take samples so these soil samples can be tested for expansiveness. These samples will show how expansive the soil is and at what depths. The geo-technical engineer provides a written report on his findings.