Get the meassuring protocol from him. It should state the NOx as well as the SO2 and the CO (and if possible the CO2) values. These you have to compare with the manufacturers values about your boiler. If you have an oilburner(?) then it is very important to get the sooth output meassured as well. This is done usually with a piece of filter paper that turns black (more or less) when exposed to the fumes. The blackishness is compared with a standardised chart and a mark is given by the protocollant/plumber.Keep the filter and the protocoll for future references, to compare it with the next meassurement.