It should be easy enough to compile weather statistics to show that forecasts of blue skies and sunny days are nearly almost always wrong these days and compare the accuracy of such forecasts with earlier times. Apparently meteorologists now need to incorporate aviation schedules in their forecasts. But then they’d probably discover that many of these flights are not scheduled and they’re not over any established routes between airports or military bases.
Or the casual observer can simply note when high-flying and contrailing planes are making turns (even U turns) in the sky hundreds of miles away from any airport. I’ve seen these myself.