The venerable JPs had a UUPI - an ultrasonic undercarriage position indicator which gave a short burst of beeps when a transmission was made with landing gear down. Inaudible to the pilots, but picked up by ATC - so if no beeps were heard, a 'check gear' call would be made.
I can recall only one of my colleagues making a gear up approach and getting a red flare. He did the same thing in a Hunter...and eventually became a navigator.
No doubt with progress, such things are no longer considered 'cost efficient'? (UUPIs, I mean, not navigators. But then again....
)