You would have to go and measure it as the instrument passes overhead.

You can use spectroradiometr..

Exactly what are you looking for? Minerals? Plants? Roofing materials? Suspensions in water? And at what scale and resolution? And where?

The 'ground truth' can be established by many different methods and is aligned to the targeted phenomena. And so, IMHO, can be based on or triangulated from lots of different methods, and sometimes an actual site visit isn't needed if those sources have a high accuracy for your target, and they are a representative sample.

Thanks Michael John Patrick and Dharmendra Singh

By field equipment, some maps for the same area also can help, previous projects,...etc.