Where Eagles Dare

In the winter of 1943â44, U.S. Army Brigadier General George Carnaby, a chief planner of the second front, is captured by the Germans when his aircraft is shot down en route to Crete. He is taken for interrogation to the Schloss Adler, a fortress high in the Alps of southern Bavaria. A team of Allied commandos, led by British Major John Smith (Richard Burton) and U.S. Army Ranger Lieutenant Morris Schaffer (Clint Eastwood), is briefed by Colonel Turner (Patrick Wymark) and Admiral Rolland (Michael Hordern) of MI6. Their mission is to parachute in, infiltrate the castle, and rescue General Carnaby (Robert Beatty) before the Germans can interrogate him. MI6 Agent Mary Elison (Mary Ure) accompanies the mission in secret, her presence known only to Major Smith.
Early in the mission, two of the operatives are mysteriously killed, but Major Smith is unperturbed, keeping Lt. Schaffer as a close ally and secretly updating Rolland and Turner on developments by radio. After seeming to give up and allowing themselves to be captured, Smith and Schaffer (being officers) are separated from the three remaining members of the group â Thomas (William Squire), Berkeley (Peter Barkworth) and Christiansen (Donald Houston). Smith and Schaffer kill their captors, blow up a supply depot, and prepare an escape route for later use before hitching a ride on top of a cable car â the only approach to the castle. Mary, posing as a maid, is brought into the castle by Heidi (Ingrid Pitt), a deep-cover agent working as a barmaid in the nearby village; Major von Hapen (Derren Nesbitt), a Gestapo officer whom Heidi has been cultivating, becomes infatuated with Mary. Mary sets a rope through one of the castle windows for Smith and Schaffer to climb.
Carnaby's interrogation, carried out by General Rosemeyer (Ferdy Mayne) and Colonel Kramer (Anton Diffring), is underway when the three other prisoners arrive and reveal themselves to be German double agents. Smith and Schaffer intrude, weapons drawn, but Smith then forces Schaffer to disarm and establishes himself as Major Johann Schmidt of the SD â the SS intelligence branch. He exposes the true identity of "General Carnaby": Cartwright Jones, an American Corporal. He also claims that Thomas, Berkeley and Christiansen are British impostors. To test them, Smith/Schmidt proposes they write down the names of their fellow agents in Britain, to be compared to the personal list in his pocket (having discreetly shown the name of Germany's top agent in Britain to Kramer, who silently affirms it). After the three finish their lists, Smith reveals his own to Kramer, which is in fact blank. Kramer realises too late he has been bluffed but Smith and Schaffer re-secure the room, the former finally revealing the mission's true objective: the lists of names the agents had provided.
Mary, while preparing the explosives, is visited by von Hapen; he takes her to the castle's cafe and persuades her to recite the tale of her assumed identity. Finding faults in her story, he investigates and happens upon the scene of Carnaby's interrogation just as Smith finishes his explanation. Von Hapen puts the room under arrest, but is distracted by Mary's entrance, enabling Schaffer to draw a hidden pistol â killing von Hapen and the other German officers. The group then makes its escape, taking the three agents as prisoners. Schaffer sets explosives to create diversions around the compound, while Smith leads the group to the radio room, where he informs Rolland of their success. They then head to the cable car station, sacrificing Thomas as a decoy. Berkeley and Christiansen attempt their own escape in a cable car, but are thwarted and killed by Smith. The group eventually reunites with Heidi on the ground, boarding a bus they had prepared earlier as an escape vehicle. With enemy soldiers in hot pursuit, they battle their way to the airfield and finally escape on a disguised extraction plane, in which Col. Turner is waiting for them.
Smith briefs Turner on the mission, and reveals a suspicion he and Rolland had shared since before the start: that Turner is the Nazis' top agent in Britain, which had been confirmed by the late Colonel Kramer. Turner had been lured into participating so MI6 could expose him, with Smith's trusted partner Mary and the American Schaffer (who had no connection to MI6) specially assigned to the team to ensure the mission's success. To avoid a trial and execution, Turner is permitted by Smith to save face and commit suicide by jumping out of the plane without a parachute. Schaffer then jokes with Smith that the next mission should be "an all British operation".

Question: How were they tested?