Your health care provider will likely diagnose corns and calluses by examining your feet. This exam helps rule out other causes of thickened skin, such as warts and cysts. Your health care provider might confirm the diagnosis by paring away a bit of hardened skin. If it bleeds or reveals black points (dried blood), it's a wart, not a corn.
Treatment for corns and calluses is the same. It involves avoiding the repetitive actions that caused them to form. Wearing shoes that fit and using protective pads can help.