They do flock testing for various diseases and certify your flock XX whatever disease free. Typhoid and Pullorum mandatory, but they do other testing also.
In the National Poultry Improvement Plan, a farm is certified as free of Pullorum Typhoid, Salmonella Pullorum by a simple blood test done on each and every bird and swabs from your incubator(s). Many states also offer or integrate Avian Influenza testing as part of their program.