Track Sagging Floor Over Time
Cheap hand laser pointer will do the job. Mount it to a flat piece of wood, or whatever. Duct tape will work fine Put the thing on the floor and lightly mark where you put it. Turn the laser on and mark where the beam intersects the wall.
wait six months and repeat. Is the mark on the wall in a different place? Then you've likely got sagging. It's easiest to figure out what's going on if you mark the wall at several different locations with the laser in one spot; the repeat the same a few times with the mounted laser in different spots.
You needn't worry about your laser mount being level, as long as its the saame each time.

You can buy a laser level at a big box store, but it's a lot more expensive and does exactly the same job as a $1 cat laser yopu can buy at most dollar stores.