For instance, walking about the place. Whether a village in Maine which got its serial numbers filed off for the work, or in Boston. Where walking about found a tower that they had no idea what it was. The householder let them get a look and even climb, and they deduced it was a World War II watch tower, for keeping watch over the harbor.
Then in Maine, the biggest challenge was the rock. This village had actual sandy beaches -- most Maine coasts are rock -- but it had a rock on the end and once upon a time there had been a battle there, and it had ended when the tide came in and everyone had to retreat. She went climbing on it despite the jagged edges and the slippery seaweed on the level parts.
Filing off the serial numbers was a common topic. One author had taken the key with the freshwater well, making it a valuable stop, and renamed it to avoid various problems. You really want to avoid real locations when talking of buried treasure in caves. . . .
One author had crossing a river on the ice. Now, this river moved along at a good clip -- 6 mph IIRC -- which meant the ice never was safe, and also that it was not level, but great jagged chunks of ice to be clambered over. He hadn't done it for research, but it was one of those things that turn into research after the fact.
Another had talked to spelunkers about the caves of Florida (which is all limestone and full of caves). They took her caving. She had thought they would go into the mouth of a cave. What they did was pull open a manhole. She had to lower herself and then crawl though a narrow passage to the real cave. She could get the claustrophibic effect very nicely, in her work.