O.K., here's what I did.
I placed the radio tower and named it: tower
I then made a trigger with: tower getdammage >1
in its condition field and in the Effects Text I wrote: It Worked
Next I blew it up with a stachel charge.
"It Worked" appeared just as it should have.
So, "tower getdammage >0" should work for you. "tower" is whatever you want to name the radiotower and the "1" is how much dammage it takes before setting off the trigger. "0" is healthy and "1" is totally destroyed. So ">0" means anything except completely healthy.
Hope this helped.