Well it would be obvious in the sense that for buildings in grassy areas, you would see plants spouting up around it, simply selecting those plants with the “destroy” task will have a hearthling go out and destroy them. Then it’s done. The wind decay is something like weather, and as I said, will only do mild to severe damage in infrequent periods (aka a bad wind storm one day). Water decay may end up being the one you’d least like, as the only way I can imagine having it visually appear is the walls of a wooden house slowly turning a dark or green color and stone houses forming small cracks.
There could also be an way to have decay appear in the notifications. Such as when a vine has grown two voxels high on your dining hall, a notification will appear that says “The Dining Hall is decaying!” or something similar. You click the notification, and it zooms into the decaying area, even rotating the camera in case you’re looking at the wrong angle. You can then select the vine and order it to be destroyed by your hearthlings.
Also, another thing that would decay differently would be roads. Yes I’m looking at those of you who would simply surround a house in a one-voxel wide road to fight off plants growing on it. They’re not safe either, and actually would decay faster with your Hearthlings running on them. Cracks and holes would appear on a single voxel of road after a certain amount of time (let’s say 60 in game days) or a certain number of steps (let’s say 700) occur on it is reached. If one does occur, both are reset (so the decay doesn’t double in severity if both requirements happen to be met within a very short time frame).
Above I had mentioned “mild” and “severe” decay. This doesn’t just apply to the amount of space in decay, but to each voxel as well. Let’s say you have a single voxel of wooden wall set up in a forest. After about a week or so of just sitting there with no one messing with it, a little vine sprouts next to it. The vine grows naturally, taking about 5 or 6 in game days to grow up into a mature vine and begins clinging to the voxel. The current decay level of the voxel is at “1.” The voxel took 13 days to reach this decay level. However, instead of taking another 13 days to reach the next level, it only takes 9. The vine grows thicker, wrapping it self around the voxel and pressing down on it, and the Voxel has reached decay level “2.” The vine continues to grow thicker around the voxel, surrounding it in more and more of the vine as 7 more days go by. The voxel is now in decay level “3,” and is considered severe. This is when the decay level takes much longer to occur due to how severe it is. It took a total of 29 in game days to reach this level of decay. The vine thickens, surrounding the voxel more and more until finally nothing but the vine is left. This is decay level “4,” and the original voxel cannot be recovered, it can only be replaced. After this the decay level will not go up any further. The vine itself will no longer grow, and will rot so long as another new voxel is placed next to it for it to attach to.
Vines can also grow up cliffs and act as natural ladders, as an added bonus. So they’re not all bad, just don’t let them grow on your buildings.
Now if there are multiple voxels, the decay would have to reach decay level “2” for it to spread to the next voxel.