This could be cancerous, allergic, due to infection, inflammation, or a foreign body. There are certain clues you can get about the nature and origin of a lump from it's appearance. In general, lumps which grow quickly, are irregular in appearance and have ill-defined margins are regarded more suspiciously. However, the only way to conclusively determine what it is composed of is to have it tested by a veterinarian. Initially, your vet will do a fine needle aspirate. This is when a needle and syringe are used to suck out a sample from the lump. The sample is mounted on a slide, stained and examined under the microscope. Depending on the results it may be necessary to do a biopsy. This is a more accurate test as it tests a larger sample of tissue. The tissue is usually sent away to a specialist laboratory for examination.