A "zero test" or "zero gun test" is used to check that the automatic timing system starts and captures properly. Typically, a starting gun (with the starting sensor) is placed flat on the finish line and fired. The timing system should capture the flash of the gun at zero seconds. The zero gun test is required for IAAF running records. During a meet, the test should be performed once a day on each of the finish lines that will be used that day.
It's a good idea to place a scrap of cardboard under the gun, or your nice white finish line will get covered with black power from the gun. Place the gun so the flash will be close to the leading edge of the finish line. (For closed barrel guns, place the chamber near the leading edge of the starting line.)