Ok...another whats it worth....a "340" this time.

You might be able to get an arm and hand under the carpet at the firewall. You might be able to feel at least some of the floor from there.

Undercoating might make it more difficult to discern rust from underneath, but if the floor pans have 30+ year old rust, the undercoating would likely have holes or cracks in it by now. You could try tapping on it with a hammer handle and listen for a drastic change in the sound.

Light rust might be impossible to discern without pulling back the carpet, though.