Some of the major symptoms of arthritis in dogs are: inactivity, some dogs may not move all day long, hesitation to jump or perform some movements, limping and lack of flexibility. May also include: lethargy, your dog will not be as cheerful as usual and will not initiate games or will not want to play or go for walks. Irritability, particularly if the affected limbs or joints are touched. You may observe this when you try to groom your dog or massage him. Your dog will not let you carry out routine grooming and may become overly aggressive when touched. Lastly, changed sleep patterns, more sleeping throughout the day and may be awake at night due to pain.
Your dog will get supersensitive, especially in the hindquarters. The only way to be absolutely certain is to take the dog to the vet. When the dog I had as a kid got it, it was hard for her to go up & down stairs & jump up in my lap. We also had to be careful how we picked her up. My mom had to give her a baby aspirin when she went to the groomer, too.
It usually is most noticeable in their back legs, when they try to get up you will see it become more and more difficult for them...if you don't want them to suffer then that is when you should take them to the vet and be a friend.