The Germans didn't get to see it at first. When a certain Hans von Greiffenberg got to visit the Mechanized force in the 1930s, the Americans who would be dealing with him had a huddle to decide how much their visitor would see:

Before their arrival, Van Voorhis discussed with Chaffee and Grow what they should tell their guests. It was decided to keep the discussions open except those on the .50 caliber machine guns. They were secured from the German visitors...

(Hoffmann, Page 173)

The result?

Van Voorhis recalled that the Germans were not particularly interested "in our equipment", but more interested "in our views on the proper tactical and strategic employment of mechanized forces."

(Ibid.)

Its good to put the situation in context. Its the mid 1930s, a 37mm cannon in a tank is considered on the higher scale of high velocity weaponry still. The .50 BMG can still defeat most armored vehicles at combat ranges at this time, was cheaper, smaller and easier to train with than a 20mm. Unsurprisingly in this context the Americans were keen to keep it hidden.

However, the Americans were very much playing catch-up, and really wouldn't surpass German and British development until the M4 series of tanks was placed into production. Unfortunately, this also led to a bit of doctrinal delay in what their equipment can do. As late as 1940, some individuals (Patton being one of the most prominent examples) still believed the .50 BMG and 37mm to be adequate anti-armor protection for their combat vehicles - an amazing conclusion given that they were already beginning to maneuver with 75mm armed M3s in the Carolinas.

Several years after the first visit by Germans, the Americans were again visited by a German observer, a von Schell:

[Von Schell] examined the Combat Car M1 and observed a tactical exercise in which the 1st Cavalry...participated. Afterward, von Schell made a number of comments...he found the .50-caliber machine gun used on the combat cars as an anti-tank weapon impractical, arguing that in the next conflict the cavalry could not avoid heavier tanks in an infantry fight. In this combat environment, he argued, the .50 calibers were useless....

(Page 208)

American Officers like Crittenberger and Chaffee concurred. Unfortunately branch limitations meant that "Armored Cavalry" could not have true tanks carrying self-propelled tanks of a high caliber, and had to force Congress's hand with the creation of a new branch, the Armored force. The usefulness of the .50 Cal for recon by fire, 'painting' targets for other vehicles, emergency anti-air defence and general suppression fire however, was rated highly by all who saw it. Its flexible, multi-purpose caliber kept it in service as a commander's mount or AA mount across all vehicles.

Source: Through Mobility We Conquer by George F. Hofmann