This is very hard to do, in fact from the ground I would expect that it is practically impossible. You might say that observing the Aurora Borealis would constitute proof that a magnetic field exists, but it would depend on just how much detailed observation you bring to bare. You could establish that auroras favor a belt or 'oval' at a latitude of about 68 degrees. This would be hard to understand physically if you only understood Newtonian physics involving gravity. You could then deduce that some kind of electromagnetic steering process was involved, and this would lead you to a magnetic field. But as a scientist you would already have equipment to detect such fields, unless you had never heard of electromagnetics. This was the situation around the time that Ben Franklin was experimenting with static electricity.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY Education and Public Outreach program.