1. It is NOT typical for rectangular front to rear floor ducts to BOW.
2. You usually determine the condition of the floor duct from just removing each floor vent cover and use a mirror and flashlight to look forward and aft between the vent covers to see if and where the "bowing" has happened.
3. If you have a bow, you can make a shim to slide in the floor duct covers(if the covers are installed with the long side front to rear and not side to side. Usually you can use a flexible type wood shim to get into and push to the bowed area to unbow that area.
4. As another posted, the other way is to remove the bottom frame plastic cover and that will reveal the ductwork except if you have fresh/black/gray holding tanks below the ductwork. Doug