Your cam lobes are in constant contact with the valve lifters, thus they will always look polished or shiny... now when you need to worry about the camshaft being worn is if there is no lobe at all on that shiny part, thus is looks perfectly round.. I've never actually seen this happen though in a good running engine.. cams are made of some pretty tough metal

also, if you're really worried, change your oil and check for metal shavings.. if you have metal shavings, you may have a lot more problems than a worn camshaft lol