True corking is where the cork itself is actually contaminated which then causes the wine to react with it and go bad. You should be able to tell the wine is bad from when you smell the cork which will be the first thing the wine waiter should do. When you're given the wine to taste again it should be obvious from the smell of the wine and certainly from the taste. A mouldy small/taste is the giveaway.
A dried out cork from incorrectly storing wine upright will cause the wine to go bad if air has got into the bottle but strictly speaking the wine isn't corked.
A screw top or plastic cork should not result in corked wine which is one reason for their increasing frequency even for expensive wines. New World wines have also embraced this form of sealing to a greater extent than Old Works wines although this is changing.
