Mix the gasses with oxygen. It will cause any NO to be oxidized up to NO2. Then N2O4 can be condensed out as a liquid and the excess O2 can be pumped off. You will know if you have pure N2O4 because it goes from a red-brown gas to a clear solid. If you get a blue-green color, you've got NO in there. Then let the N2O4 warm back up and you'll get the NO2 back again.