No one wants their pet to suffer, so this is a really common question. It is often obvious when an animal experiences acute or sudden sharp pain (for example from a bee-sting or a broken leg) as they display many of the behaviours that we associate with pain – they may vocalise or yelp, turn towards the source of pain or try to “escape” it.
Chronic or long term pain, however, is far more difficult to identify, and is probably much more common. The most frequent cause of chronic pain in our pets is arthritis. Arthritic animals may be in a considerable amount of pain, however owners often put it down to "just getting old" as they believe that if it was bothering them they would cry out.
A good way to think about how chronic pain affects animals is to think about how it affects us. For example, if you have experienced back pain, you will know how debilitating it can be. For someone with chronic back pain, it is highly unlikely that they would cry out. Realistically they would mostly grin and bear it – they might avoid certain activities such as lifting heavy objects, or change the way they do things, and you wouldn't blame them if they were a bit grumpier than they otherwise would have been!
Now relate that to a dog with chronic pain associated with arthritis. A popular misconception is that they would yelp if it was painful but actually they mostly just grin and bear it too. They may avoid painful activities like jumping up into the back of the car, or change how they do certain things. Even something as basic as standing up from lying down can become a difficult task – it may be very slow and is often particularly bad when they have been lying down for a while, or have overdone things the day before. The pain associated with these basic activities often means that they would rather lie around sleeping than be up and active - hence the reason that owners think they are "just getting old". Certain dogs will become "grumpier" too, meaning they are more prone to biting if hurt.
As with humans, there are a lot of things that can be done to reduce this chronic pain. One of the most satisfying parts of our job is treating pain in an old arthritic dog who the owner later reports has begun acting like a puppy again! So next time you think your dog is "just getting old", take another look and see if you can identify any signs of pain as wouldn't it be great to have them acting like a puppy again?