Ability to cut a clean edged piece of notebook or slick typing paper is the indication. It will dull a blade pretty quick, but you will learn when your blade is really sharp. Paper that has a high fiber content, often rag or linen, let's an edge get a hold and isn't as good of an indicator as a 'slick paper' like notebook paper. Durability of an edge is mostly related to it's angle--(metal hardness, chromium content etc. have a lot to do with it also, but) if an angle is really shallow and sharp, i.e. a scalpel blade or razor blade, the angle is really shallow, but the edge(sharpness) is very short lived, (scalpels are replaced not sharpened). A good angle for a hunting knife is around 25 degrees, a good blend between sharpness and duarability.