How incoming enemy aircraft were detected before the invention of radar…
Before electronic RADAR, acoustic listening devices were like giant mechanical ear trumpets which could locate sounds and even calculate distances by bouncing sound waves in exactly the same manner that SONAR works in water. Ear trumpets themselves were only fully replaced by electronic devices in the middle of the 20th Century, because of their conspicuous nature, they were often hidden in anything form beards and wigs to table ornaments.