It's not that hard, and most of the smaller AM and FM facilities I've seen (ones where snakes nest on top of the transmitter and wild turkeys try to chase off the engineer the one time a year the tower gets a visit) still have some sort of monitor. Plus... especially with AM, there's always at least one old folk sitting there with it on 24-7. It's usually a race between the engineer and that older person to see who calls the studio or owner first!
I have called my local station about dead air a couple times. I was the first to tell them. One time in recent memory, it went on for so long that I forgot the radio was on when they came back.
