How much Rust is to Much?

[kaps4r]

I'm currently looking into buying a '05 Limited V8. It only has 55000 miles on it and was driven as a commuter. It has experienced very little hard driving, or towing. Unfortunately the owners were not real big on aesthetic upkeep. It was rarely washed and there were no precautions taken towards rusting, especially in the back end. Are there any quick easy methods to determine if the hitch receiver and rear frame elements are structurally sound?

Thanks!