Sinkholes form when water dissolves minerals in the bedrock, leaving the structures on top of the ground sitting precariously on loose soil that can quickly wash away.
If a sinkhole is in the process of forming, the structures on top of it can give a hint to its existence: for example, slumping or sagging fence posts and trees that start to lean. More subtle signs can include doors and windows that don't close properly.
Heavy rains can trigger the collapse of a sinkhole that had been slowly forming. When rainfall collects in an area where water previously did not collect, that is another sign that the land is subsiding.