I vote for visual observation. I've been watching chemtrails for the past few years and seen them evolve from very obvious patterns in the sky, to spraying that eventually looks like a form of natural cloud after some time has elapsed.

The debunkers say that we are just mistaking regular plane contrails for chemtrails (deliberate spraying of some chemical(s) for some publicly unknown purpose). This is obviously and demonstrably untrue to anyone who watches for any length of time. When the spraying is being conducted the skies are full of jets criss crossing each other and leaving these trails behind. If these were regular flights we would see them ALL the time, certainly around the same time of day. And yet, even this past week we had one day where the sky was so clear there were no clouds - just blue sky. And NO contrails or air traffic of any kind except for the occasional jet. The chemtrails are NOT residue from any regularly scheduled flights.