If you were to report it to someone, you would probably want to use a standardized test (ASTM D3237 by AA), ASTM = American Society for Testing Materials. University libraries typically carry the ASTM collections. If you are in Europe, it will probably be in an ISO (International Standards Organization) or DIN method (Germany only).
There are other approaches to determining the lead content in gasoline. Keep in mind that most places phased out the addition of tetraethyllead as a gasoline additive in the 1970's. I believe the only countries that still use "leaded" gasoline are in Africa.