How did the west find out about STOL performance of the Soviet aircraft?

This comes from a top secret recording describing what happened at a soviet cockpit during approach to JFK at IFR conditions:
Captain : "500 ft. low visibility, runway seems to be short. Co-pilot, lower flaps to 30 degrees..."
Co-Pilot: "Flaps down at 30.. check."
Captain : "300 ft, low visibility, runway looks very short... Co-pilot, lower to 45 degrees !!!"
Co-Pilot: "Flaps down at 45...check."
Captain : "100 ft !!! Runway very short !!! "Co-pilot, lower flaps to emergency - 90 degrees down!!!
Co-Pilot: "Flaps down to 90 degrees ready to land...check." Aircraft lands and breaks immediately...
Captain : "*wipes off the sweets and sighs*... "Ohh they make the runways so short in the west...."
Co-pilot: "Yes...*points out the window*...but they are so wide..."