In order to prove treason, the prosecution needs either a confession or two witnesses testifying to the same "overt act" by the defendant. ... While testimony from two witnesses is required to prove the overt act, the intent to betray can be proved in the same way as intent for any other crime.

Who decides on the Punishment for treason?

The Congress shall have Power to declare the Punishment of Treason, but no Attainder of Treason shall work Corruption of Blood, or Forfeiture except during the Life of the Person attainted.

What is Punishment for treason?

Whoever, owing allegiance to the United States, levies war against them or adheres to their enemies, giving them aid and comfort within the United States or elsewhere, is guilty of treason and shall suffer death, or shall be imprisoned not less than five years and fined under this title but not less than $10,000; and ...