At least once a week, run your hands all over your pet's body. You will usually feel a lump, bump, or rash before it is visible, especially in long-haired dogs and cats. This can be a time to groom your pet and they will enjoy the physical contact. Check all those hard to see places- armpits, underbelly, and under the tail.
Any new lumps that you find should be checked by your veterinarian. It may be a harmless swelling, but if not, treatment is always more successful when started early.