What details lead you to infer the men Rainsford meets have shared a military past?

Ivan's response to Zaroff's arrival on the stairs helps us infer that the two men have a shared past in the military.

The man's only answer was to raise with his thumb the hammer of his revolver. Then Rainsford saw the man's free hand go to his forehead in a military salute, and he saw him click his heels together and stand at attention. Another man was coming down the broad marble steps, an erect, slender man in evening clothes. He advanced to Rainsford and held out his hand.