I would base my judgement on the appearance of the brake fluid when you flush and bleed your brakes. That's about the only way to gauge the condition of the insides of your brake lines. If the fluid you flush out of the system has little bits of junk in it, those bits of junk are coming from the insides of either your master cylinder, wheel cylinders, or lines. You can open up the wheel cylinders and have a look inside farily easily, the master cylinder a bit less easily, but you can't really see the insides of the brake lines.

I agree that the original materials are probably tougher than new steel lines.