If your foot falls thru there is a problem.
Lot of posts in the archives about soft spots. Vinyl tile is a problem as small damage won't show up, with carpet it is easy to use an ice pick determine damage. Study the archives and find where specific models have been known to leak, windows, door, vents, and you might be lucky to find one that has been stored inside all its life and not have any. I have a moisture meter for wood and it can show where a problem exists without damaging surface, but it just shows where it is wet or changed from solid plywood even tho you can read numbers changing, nothing like visually seeing damage.