[header] How to diagnose skin masses on dogs [title] Feel the skin mass. [step] Start by using clean hands to gently feel around the skin mass. The skin mass may be located on your dog's belly or trunk, though they can appear anywhere on your dog's body. 
Note if the skin mass feels soft and moveable under the skin. If this is the case, the skin mass may be a buildup of fatty tissue, known as a lipoma, which is not life threatening.