Keep track and report any of these signs to your veterinarian before it becomes serious.

Just not acting like himself/herself
Interacting less often with family
Responding less often of less enthusiastically
Changing in behavior/activity level
Having difficulty climbing stairs
Having difficulty jumping
Exhibiting increased stiffness or limping
Drinking more often
Urinating more often
Changing eating patterns
Noticeably gaining or losing weight
Losing housetraining habits
Changing sleeping patterns
Becoming confused or disoriented
Changing hair coat, skin or new lumps or bumps
Scratching more often
Exhibiting bad breath/red or swollen gums
Showing tremors or shaking