How did French intelligence learn about German preparations for the Verdun offensive?

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French soldiers in uniform rest and eat on a grassy field behind the front lines during the Battle of Verdun in World War I, with gear and rifles stacked nearby. — Battle of Verdun French soldiers resting away from the front line at the Battle of Verdun, 1916.

French intelligence learned of German preparations for the Verdun offensive through aerial reconnaissance and ground-level observation. As early as January 1916, French airmen detected German preparations for the attack. Subsequently, on February 11, 1916, a French intelligence officer identified a significant buildup of German troops on the right bank of the Meuse River.1

Despite these early warnings, the French high command was largely preoccupied with its own projected offensive plans, causing the intelligence reports to be initially overlooked.12