[Tarpon]

For far-off observations, it's done by astronomy. With Mars it done with orbiting satellites.

The weakening magnetic filed is one component, if it actually goes to zero as the poles switch, effects move into the unknowable column.

One recent study has shown that cosmic rays from exploding distant stars have an effect. There was one recent paper that said had a recently observed explosion been directed at Earth, all life on Earth would have been extinguished. A small probability, but nonetheless real.

Mark Twain:

"It’s not the things we don’t know that fool us. It’s the things we do know that ain’t so."

[Vicomte13]

"It’s not the things we don’t know that fool us. It’s the things we do know that ain’t so."

So very true.

I am a master ultracrepidarian myself.