Since the 1970s satelites have been used to compile ice charts for the Arctic and Antarctic oceans. These work by monitoring the microwave emission of the ice or ocean or by firing microwaves down at the ice and listening for the faint returns. Microwaves are especially useful as they pass through clouds, and don't need light from the sun, so work in all weathers and all seasons. Different types of ice scatter and emit the microwaves differently, and ice is very different from open water. By following what happens year on year, and applying algorithms which seek to distinguish the various types of ice, it's possible to make maps of the extent, surface cover and (less well) the likely age of the ice and so some idea of its thickness.
In a recent GRL article "Perennial pack ice in the southern Beaufort Sea was not as it appeared in the summer of 2009" Barber et. al. went sailing through the ice in the sector of the Arctic near Canada early in Autumn, just as the ice was starting to form again. Charts produced from RADARSAT suggested they'd encounter thick multiyear ice but they found rotten ice, formed from ice that had almost disintegrated in the summer, to the point that it had a structure like Swiss cheese, and had since grown a thin cover of ice inside the holes. It turns out that this mix of ice looks like old ice, but is really better classified as weak young ice.
This shows in the most compelling way why we need to spend a lot of (your) money and (our) time and energy getting out and poking bits of the world to see how they work.