@BattleDwarfGimli If they are spraying there's no real way for someone to get a sample at altitude.

When they take off the spigots would be closed.
My guess would be when they land there would still be traces of the chemicals possibly leaking from the sprayers not being perfectly closed or coming off the plane's skin, because I would think it's turbulent up there enough some would stick to the outside of the plane.

Wait for them to land and sample a few times before they come in and then a few times during and after landing and hope you pick up some trace amounts of something.

A jump in anything at the landing sample and after indicates something. There's very sensitive analysis equipment available. Of course it might require you know what you analyzing for in the first place.

Just spit balling ideas here.