Some behavioral and physiological signs that your pet may be ready to pass include:
Extreme fatigue or loss of energy
Loss of bowel or bladder control
Change in appetite or loss of appetite
Erratic breathing
Is in pain
Hides or withdraws from people
Weight loss
Depression
Keep in mind each animal is unique and they may not display signs usually associated with pain such as crying or whimpering. They may also continue to eat and drink despite pain.  You know your pet best, so please talk to your vet if you notice any abnormal behaviors and they will be able to offer insight based on your pet's symptoms and medical history.