How did George Washington find out Benedict Arnold was a traitor?

Answer 1

When he received word that Benedict Arnold was going to willingly surrender West Point to the british. When George Washington learned of this, he and his men moved toward West Point and Benedict Arnold being the cowardish traitor that he was.......ran.

Answer 2

He accidentally alluded to it on his Facebook page...

NEW RESPONDENT

The Americans captured a British Major, John André traveling in civilian clothing through the New York Colony's Highlands near Tarrytown outside the British lines. under the assumed name of John Anderson. They found on his person a pass signed by Arnold and the detailed sketches of the network of fortifications of West Point

defenses. The documents were consigned to Washington who deduced that Arnold was a traitor.