How did the north spy on the south in the civil war

At the start of the war, the Union didn't have a very organized spy network. Although they had a lot of spies, they were usually sent out by individual generals or leaders in the government. The information wasn't communicated very well or passed on to the people who really needed it. The Union had a distinct advantage in the war as they gained a significant amount of military intelligence from slaves and former slaves.

As the war continued, the lead generals of the North had a spymaster who would organize and gather the information from the spy network.

Secret line

Explanation:

The Confederate spy networks were more organized than those of the North. They mostly used people who worked for the army or the government in the North, but who secretly wanted the South to win. They passed information across the border using a system called the "secret line."