How is cutting cardboard a way to tell if a knife is used? Not all knives come razor sharp and that has nothing to do with if it's used or not!! You sir are the ignorant one. How to tell if it's used...try looking it over, look for signs of use like scratches on the handle or blade. Honestly though those are things that can slip past QC at any factory so there may not be a sure fire way to tell 100% but cutting paper and cardboard are NOT ways to tell. But like some of us said. You did 100% use the knife and now want to return it for the same thing.