A typical sign of patellar luxation is when the dog skips on a back leg. He may be perfectly mobile and not in pain, but every now and again he skips a step. He may also carry a back leg for several steps before putting it back down again.  This is due to the mechanical locking of the knee joint due to the faulty anatomy. After a few steps, things tend to shake back into place and the dog continues to walk as normal. Patellar luxation occurs in the back legs. This means your dog may move all the weight into the front legs. It is possible in some dogs for both legs to be affected by patellar luxation. If this is the case, the dog may look like he’s doing a bunny hop with both legs together. If your dog has the condition in both legs, he may have difficulty jumping up onto furniture or low surfaces. In severe or long term cases, the dog may develop secondary problems. One secondary problem may be arthritis in the joint. There may be swelling or stiffness in the leg or joint. If the dog is using the leg less, he may also develop muscle wastage on that side, which means the muscles on the affected side will be smaller than the ones on the other side. Your dog may also exhibit lameness on the affected leg. Lameness means your dog won't have the ability to walk on that leg. A dog can get patellar luxation at any age. Many dogs often present symptoms as a puppy because it is a problem with the muscles of the legs. Dogs can start presenting symptoms at at eight to 10 weeks.

Summary:
Look for skipping on the back leg. Watch for difficulty jumping. Monitor for other leg problems. Recognize the symptoms at any age.