Many buildings constructed before 1978 used lead paint. If renovations or painting is done and lead paint had been used previously, it would be covered with new paint thereby encapsulating it. Only if its disturbed will it become friable, not unlike asbestos.
A tool / test equipment called an XRF gun shoots a radioactive cobalt beam through the surface being tested. This radioactive source excites the lead molecules in any lead paint which bounce back into the test equipment and gives the technician the exact amount of lead in the area being tested.