Which steel lines do you mean? The lines along the frame rail can get rusty(mine have rusted out). The lines between the lift pump and injection pump have been known to crack and leak fuel but they don't tend to rust. The same is true of your injection lines.
I assume that you are talking about the lines along the frame. You can visually inspect them but the worst part is likely to be behind your fuel tank. If they need to be replaced, I would recommend running marine fuel line from the tank all the way up to the lift pump and doing away with the metal lines.