Here's how you can tell if your cat has arthritis:
Hesitant/reluctant to jump up or down e.g. on furniture or through cat flap
Smaller jumps rather than big leaps
Less graceful movents e.g. 'stiff' or 'creaky'
Litter tray 'accidents'
Sleeping in one place more
Playing less
Hunting less
Coat changes, especially in the lumbar region. The hind end may be matted and/or scurfy
Changes in scratching
Overgrown claws which are clicking on hard floors and catching on carpets
Less tolerant e.g. towards children and/or other cats
More withdrawn and/or reduced interaction with owner(s)
Significant improvement in willingness to jump and height of jump
Significant reduction in pain on palpation
Significant and continuous improvement in temperament reported by owners