This is a great article that Dr. Dressler just published (thanks for bringing it to my attention Sally!)

How to Know if Your Dog is In Pain

They don’t want us to know when they are in pain, and they will do everything they can to hide it.

They’re afraid if we find out that they are in pain, we will think they are weak and we will leave them behind.

How Dogs Signal Pain:

• Limping ALWAYS means your dog is in pain
• Doesn’t want to be touched
• Resistance to using a certain area of the body.
• Bunny-hopping (it looks cute, but it’s not normal)
• Vocalizing (whimpering, whining, yelping) for “no reason,” particularly when lying still or standing still
• Unusual Panting
• Unusual Trembling
• Resistance to climbing stairs or getting up on furniture
• Guarding: hunching over as in a stomach ache
• Straining to urinate or defecate
• Listless or apathetic behavior
• Lack of appetite
• Lack of engagement (isolating, withdrawn, refusing to play or walk)
• Aggression

If you see any of these signs, it could mean your dog is experiencing some measure of pain, and you should get it figured out so you can address it.

Read more here