Question

How do you know the Zimmerman note was a reliable source

Answer 1

The Zimmerman note was a reliable source because the author himself, Arthur Zimmerman, admitted that it was genuine communication from the German foreign office. The note suggested an alliance between Mexico and Germany against the United State in which Mexico could regain the territories of Texas, Arizona and New Mexico.

Answer 2

The main actors in the transmission of the message were tracked, followed  and then authenticated. The British intercepted the message on the British and American transmission lines as a coded message. They partially decoded it and understood the ramification and assumed the next level of transmission would require some kind of commercial carrier in Mexico. They tracked this down and conveyed all related information to the Americans who were inflamed and began their participation in WWI.