Things like rust and pitting should stand out with the naked eye. Also check the end of the bore for cleaning rod damage. If you want to see an excessive case of this look at some military rifles and you' ll see what I' m talking about. The rifling should also appear sharp and not rounded. Of course a visual inspection will only point out abuse. A barrel may be shot out as far as top accuracy goes and you' ll never be able to tell by looking. That' s why I won' t buy used varmint or target rifles.