Read this paragraph from The Dark Game.
Another thing that changed very little in the years between the wars was the means of obtaining military intelligence. Although the use of photography and the telegraph brought some technological advances, the craft of spying on the enemy's army still relied on fieldwork. For the most part, intelligence was gathered in simple ways. Prisoners of war and deserters were interrogated. The Union army was always interested in hearing what runaway slaves had to report. And, of course, both armies relied on cavalry scouts, signal intercepts, visual observation, and captured correspondence.