When I look at the floor and side rails, I use a screw driver to stab at them and see how soft they are; don't do this unless you have permission because the current owner may freak out. The metal angle iron in the walls failed before the floors in my stock trailer, but I think that's rare. You can also tap on them with any solid object and hear if those parts and solid, but it's not as accurate because you have to know what the sound should sound like. Good shopping.