Some of them use a satellite service that will point out new structures. When I built a barn it wasn't two weeks later that I looked out and saw a man measuring it. Yep from the appraisers office. He said the satellite picked it up.
It can work both ways. A farm I bought showed some improvements even though they were bare pastures. I ask the chief appraiser if he could look on his Sat view and see that there was nothing there to save us both the time of an appeal. He agreed and let me know a couple days later that at one time there was a barn on the place but it was removed some 20 years before. They had been collecting on it all that time. Saved me a few dollars.