For starters, here’s a relatively detailed explanation of the decay mechanics:
Now, on to your questions:
I don’t know for sure. What I always do is go right up to the building and actually look at the decay timer until I can see it has been reset to its maximum value.
Yes. All you need to do is craft a repair hammer and equip it. If you’re wielding it and you come close enough to a building piece or a placeable, you will see its decay timer.
I covered that in my post about the decay mechanics, but I’ll answer the question directly: it depends on whether it shares the decay timer with your walls.
There are a couple of ways to make sure a building or a placeable shares the decay timer with another. The easiest and most straightforward way is to connect the building pieces, but there are others.
Proximity is one of the ways: for example, if you want to place a wheel of pain but you don’t want to spend resources on additional foundations connected to your walls, you have to make sure the wheel is as close to the walls as it can be. When you place it, check its decay timer. If it’s not the same as the walls, pick it up and place it closer.
Another way is to build it as a connected build first, and then either destroy or pick up the connecting pieces. That way, the game will assign the same building ID to all the pieces, and that building ID will remain the same even if you break the connection.
I hope that helps.